David Mayer's 1990 book George Kennan and the Dilemmas of US Foreign Policy has the most detailed account on Kennan's view on that that I could find, albeit a lot of that is paraphrased (pp. 152-155):
Against the ominous background of communist coup in Czechoslovakia and
Soviet military rumblings—related, as it turned out, to preparations for the Berlin
blockade—the Truman government began investigating the means by which the
United States might best support the Brussels coalition. Public opinion, as measured
in the Gallup polls, was growing in favor of a permanent American alliance with the
principal states of Western Europe, but Kennan was unimpressed, and he told
Marshall that a formal US-West European military connection was incidental to
thwarting the type of threat really posed by the Soviets. [...]
Kennan did admit that a transatlantic
security pact would probably boost the confidence of Frenchmen, Britons, and
others, but he worried that a heightened concentration on military matters would
divert attention from the urgent problem of completing Europe's economic recovery
and would prejudice all efforts to find diplomatic answers to the East-West conflict.
As it was, he lamented, too many people on both sides of the Atlantic were
preoccupied with a military balance of power on the continent. Of course, a healthy
balance was essential, but Western Europe's ultimate success and safety depended
on the outcome of its struggle for internal political stability. In other words, a UScentered
military alliance would "address itself to what is not the main danger." [fn: 69]
Once it did become certain that the United States would join some variety of
European security pact, Kennan hoped to limit its geographic scope to those states
bordering (plus Luxembourg) on the north Atlantic. [...]
Kennan also worried that, if a US-West European military organization were
established to include states east of the Atlantic zone (e.g., Italy, Greece, West
Germany), the political division of Europe would deepen and chances of its future
eradication would be slight. None of the satellite regimes could thereafter contemplate
a slow reorientation away from the Soviet Union: "Any move in that direction
would take on the aspect of a provocative military move." An eventual Soviet-US
withdrawal from the center of Europe would be foreclosed; no third force assuring a
more stable balance of global power would issue from the continent. To avoid such
an acutely strained international division, the North American and European Atlantic
powers should assure West Germany, Italy, the Scandinavians, Greece, and
other affected nations of their lively concern for the security and interests of noncommunist
Europe. [...]
Bohlen was Kennan's only significant ally on this matter (in the State Department)
and agreed that the administration should proceed cautiously before tying the
United States to a solemn security arrangement in Europe. To his dismay, Kennan's
own Policy Planning Staff voiced contrary views and during his absence (to
the Far East) in March 1948 produced a set of recommendations going beyond what
even Bevin thought necessary. These proposed that the Brussels Treaty states
expand their membership to include all of Scandinavia, Portugal, Italy, and eventually
Ireland, Switzerland, Germany, Spain, and Austria. And it would be understood
that a military attack against any one of them would constitute an attack
against the United States. In mid-April, the NSC adopted most of this advice over
Kennan's protests.
[...] In the end, Kennan resigned himself to the concept of a North
American-West European defense pact; ironically, in late 1948 he was assigned to
help devise NATO's specific provisions and institutions. He nevertheless remained
skeptical about Italy's admission and later was adamant in his opposition to the
entrance of Greece, Turkey, and (in 1954) West Germany. [fn: 75] His desire remained
for a looser arrangement relying on a US-Canada alliance that would cooperate with a European military coalition centered in Britain, France, and Benelux as circumstances
warranted.
So, yeah, in summary, he kinda opposed NATO in principle (as distracting), and he more explicitly opposed NATO including specific central or southern European states at the time.
The book has academic style footnotes, which the most relevant ones for that section are primary sources:
- SDPPSP 1948, PPS 43, November 24, 1948, p. 491.
- Ibid., p. 492.
[...]
- Kennan to W.W. Rostow, May 15, 1962, File "2-B 1962" in Box 28, Kennan
Papers; Kennan to Norman Graebner, May 16, 1959, File "2-B 1959" in Box 28, Kennan
Papers; Kennan to Robert Matteson, December 16, 1958, File "2-B 1958" in Box 28,
Kennan Papers.
I suppose that double checking those is rather difficult (especially the latter--75). I had hoped the SDPPSP might be online, but alas it doesn't seem to be, or at least "address itself to what is not the main danger" doesn't return any hits on the web but to Mayer's book.