Hi Melodie,
You don't need to be strong for anyone but him (and if he sees tears in your eyes occasionally, that is not a bad thing; I'm sure he has or would want to shed some). And you can stop listening to the outside voices that don't matter. The two most important ones are your own, guiding you as to what is right, and his, that of course comes first.
Guys have a hard time accepting life-changing illnesses as it is -- and ALS is its own species.
If you continue to show your husband without pressure, that you are ready, willing, and able to undertake any adventure or diversion he might want to try, whether a 4-minute video or a walk in the park, he is more likely to act on what he feels are his own ideas. It has only been a couple of months, and I'm sure the holidays were tough.
Going a step further -- I would try to forget the phrase"being negative." For some, withdrawal is part of the process of living before dying. It is a way of protection, of giving up expectations, and being less hurt with each loss.
Though there is much life left if/when he wants it, that has to be his choice, as does when/what he shares about his feelings. Leave him the white space for listening to whatever he can say in words or otherwise. I truthfully don't remember my husband talking about ALS, per se, past the day of diagnosis. So if that day of sharing never comes, it doesn't mean you can't have a rich life past today -- it may be more one less-planned-than-before moment of shared experience at a time.
Best,
Laurie