Yes ($8200 for 2021). Not only that, your son can open an HSA in his own name and contribute $7200, as long as he does not have some kind of disqualifying secondary coverage like Medicaid or a PPO or FSA from his employer or from his other parent.
There is a rule that spouses can only contribute the same overall family maximum ($7200), split any way they want. But Congress seems to have overlooked the situation of adult children. If your son is "covered" by a qualifying family plan (and he is), then he can contribute up to the family limit into a separate account in his own name, even though he is not the owner of the insurance policy. He might not save much in taxes, depending on his other income, but the savings would grow tax-free for future medical expenses or retirement. An HSA makes an excellent retirement account and he can contribute even if he is not allowed to contribute to an IRA due to a lack of earned income.
