Will I still be able to PCS to Alaska if my spouse and son both have HHT and they're both enrolled in EFMP?

Yes. No. Maybe.
EFMP will make that determination - that is LITERALLY what they are there for. They will look at your projected assignment, look at the available medical resources (to include community resources, not just military medical), and then make a determination on whether that location can provide proper medical support.
My GUESS, and it is only a guess, is that Alaska will have less available medical resources, and it may be a no-go. But it is JUST A GUESS. EFMP will KNOW.

Start working EFMP NOW.

Part of an oconus PCS is the efmp screening required in order to get command sponsorship. Alaska will determine if they have the efmp resources available it support your family and approve or disapprove your command sponsorship accordingly. If your command sponsorship is disapproved then you need to get with your branch manager or career counselor, depending which way you got your assignment, and work out a new assignment

I’m in a similar pool with you. With my family member in EFMP, when you go to retention you’ll have to inform the retention NCO, but HRC has a designated section for EFMP. They’ll be able to tell you if you can go or not and, possibly, offer an alternative if you’re unable to go there if the location can’t support your needs.