I understand how you feel about your son, but he is two and while he doesn't really understand what is going on, he can see that Grandpa isn't happy today. Maybe those are the teachable moments for your son to say..."gee, grandpa is a little sad today...what do you think are things we could do to cheer him up?" Then help him with a few suggestions if he can't think of any like..."Let's draw grandpa a happy picture to cheer him up", or "let's go pick a flower for grandpa", or "sometimes we can't make the sad feeling go away for someone...but we can remind them that we love them, maybe just a nice hug for grandpa and then we can leave him be until he feels better". These things a 2 year old can understand. Plus, it is acknowledging the situation in a way his age can understand. And as far as what you could maybe do, acknowledge what you see lovingly and say something like, "gee dad, I see that you are down today, is there anything I can do to help you?" If he tells you great, if not, just remind him you are there and love him and then act as normal as you can. But I think the worst thing you can do at that moment is to ignore the obvious. Just be as loving as you can and if he doesn't feel like opening his gift that day...don't make a big deal of it, say, "that's okay...you can open it whenever you feel up to it. We hope you really love it." Again, you aren't going to make him be different or "pull" him out of where he is. He will have to learn to see it and do it. The more I left my f-i-l alone and didn't push feelings with him, the more he opened up with me. In the end, when he passed, it was me that he reached for the most.
As to taking anything personal, given that you recognize something is off with him, it should make it easier to disregard the way he may handle things as "oh, there goes dad again..." in your own head. But if you can't get past taking it personal, then that is definately an area the therapist can help. Plus, by seeing the therapist with your husband, he may learn how to talk to his dad about what you both are seeing in a loving non-threatening way and perhaps he will want to get help one day. You never know. But it sounds like your husband does see it to, so give them both time. You planted the seed with your husband as to what it could be...now go to the therapist...and let the seed grow for him. He will learn and adjust to the idea. Then he may feel safe about how to take action.
Hope that helps.