Edit: I now see your husband is a retiree, so the comments about COBRA do not apply, but leaving them for someone else.
It is impossible to answer the question about whether he should enroll in Medicare now without knowing what the copays and out-of-pocket limits are for his employer plan, how narrow a network it is, whether it's an HMO or PPO design, etc. Re the deductible, I do not believe a commercial plan will prorate the deductible against another payor, but you can verify.
To avoid the Part D late enrollment penalty, he will need to have/document creditable drug coverage if he stays in the retiree plan would do that as soon as Part B starts so he doesn't have to "undo" the penalty later.
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The COBRA duration for dependents depends on a few dates.
See
COBRA Continuation Coverage | CMS.
The 29 months applies if disability is determined during or made retroactive to the first 60 days of COBRA coverage. If the Medicare eligibility date precedes termination, COBRA may run as long as 36 months.
That is one reason why terminating employment (which drives the COBRA effective date) with strategic attention to dates is important -- disability benefits and retirement benefits are others. I advise everyone, especially those with a choice of secondary payors, to contact SHIP/SHIBA and your most savvy HR benefits person before making any moves, including termination of employment.
Using a COBRA plan as a secondary payor to Medicare can be a hassle if the plan is heavily managed. You can ask your benefits person what the experience has been.