"Hired on by Searle" is not exactly true, but "accepted a position as a paid consultant to Searle's public relations firm" is true. There isn't direct evidence showing that he did any work there on behalf of Searle, though.
According to a 1986 review done by the Government Accounting Office concerning the aspartame approval process, here is a short resume for Dr. Hayes during the time in question (pg 13):
Arthur H. Hayes, Jr., M.D. Tenure from/To
Commissioner, FDA Apr. 1981 Sept. 1983
Provost and Dean of the New York Medical College Sept. 1983 July 1986
President, E.M. Pharmaceutical, A Division of E.M. Industries July 1986 Present
Senior Scientific Consultant, Burson-Marsteller Nov. 1983 Present
You can see he left the FDA in September 1983 and was hired as a consultant for Burson-Marsteller in November of the same year. Searle was, at the time, a client of Burson-Marsteller, a public relations firm. B-M has a number of other clients, and is not a subsidiary of Searle.
Hayes and Burson-Masteller both deny that Hayes did any consultation related to Searle specifically (see pg 10 of GAO report), and provided documentation as evidence:
In July 1981, former FDA Commissioner Hayes approved the use of aspartame in dry foods.
Dr. Hayes served as Commissioner from April 1981 to September 1983. After he resigned as Commissioner,
he was the Provost and Dean of the New York Medical College until July 1986 when he became the
President of S.A. Pharmaceutical, a division of E.M. Industries.
During this time, he was also a Senior Scientific Consultant with Burson-Marsteller.
According to documentation provided by Burson-Marsteller, Dr. Hayes has not consulted with Searle
through Burton-Marsteller. Dr. Hayes advised us that he has never been asked by Burton-Marsteller to provide
consulting services to Searle and before becoming FDA Commissioner be did not perform any services for Searle.
Dr. Hayes added that he has met only once with Searle officials; the meeting, in 1983 while he was still FDA Commissioner,
was attended by both FDA and Searle officials to discuss FDA's decision relating to aspartame's use in soft drinks.
This meeting was recorded in FDA'S Public Calendar (see p. 6).
We found no record of Dr. Hayes contacting FDA after he resigned as Commissioner in September 1983.
In this regard Dr. Hayes advised us that he has not represented Searle or any manufacturer before FDA,
nor has he discussed aspartame with any FDA officials since he resigned as Commissioner.
Current FDA officials confirmed that Dr. Hayes has not discussed aspartame with them since he resigned.
Other than his meeting with us on May 9, 1986, Dr. Hayes stated that he has not discussed aspartame with anyone.
Now, obviously they would have motive to cover up any dealings he did have with Searle, but the GAO seems to have accepted this at face value, as no contradictory evidence has been brought forth.