R.M.
Wow, what a difficult situation!! Of course I am sure there is a back story to this that we don'tknow about...some reason that things are so tense between your husband and FIL. Maybe your FIL would consider coming to YOU since it is such a difficult time for you to be out of town. That sounds like the logical thing to me. If that doesn't work I see 2 possible solutions
1. Your husband goes alone and you stay home with your son to get him to camp and let him have a nice smooth start to Kindergarten.
2. Your husband tells your FIL that this date that he has "penciled you in" on in calendar doesn't work for you right now..that he is very sorry but Dad will have to let him know when there is another free day in the foreseeable future that he could squeeze you into his terribly full and important schedule!! (Oooooh do I sound a bit sarcastic??? Sorry!!!)
I would talk this over with your husband, see how HE wants to handle it...and just let him know that you will do whatever he feels like is the best.
I am just sorry that the family situation is so complicated...but my guess is that this is not something that just started this past year....I bet that the relationship between your husband and his father hasn't been good for a long time.