My immediate thought was loss of HSA access, as others above have suggested.
Assuming a HDHP is what he’d select, if he has that plan on the last month of the year then he’s eligible for a max HSA contribution for the full year as long as he continues to be eligible for all of next year. Look up last month rule for details.
Of course, the value of the HSA contribution to him depends on the salary and marginal tax rate as well as whether he’ll be maxing out the 401k. Those who see the HSA as very valuable are usually maxing out 401k contributions and for the most part aren’t planning to use HSA funds in the short term (they can be used tax free decades later for eligible expenses, including Medicare premiums).
Employer contributions are worth capturing if the math works.
Ultimately it’s probably not a big deal for only a few years, and if there’s a major medical expense he will pay more with the HDHP.