3 kids, all primary age, their Dad left the marital home a few months ago and is sharing with a relative until he buys/rents. The children don’t like going to his current place, but one of them is very allergic to cats and there are cats at the house. Whenever he has been his skin and asthma flares up and he also is very unsettled, so now he’s decided he isn’t going. I spoke to my ex and asked could they do something else other than go back to his current house, which is an hours’ drive away. I suggested open air museum, trampoline park, country park, seaside. All “no.” And he said if he doesn’t want to come then he won’t see him. 
Ex is much closer to one of my other children and will focus on him anyway, which I loathe, so leaving this one at home, just exacerbated this.
I’m disappointed but not surprised about my ex’s inflexible attitude. But I’m also sad for my little boy. And from a selfish point of view I wouldn’t mind a break! Ex only has them for the afternoon every other weekend, and they don’t do over nights.
Should I force my son to go? How do I handle this legally? My thoughts are to formalise EVERYTHING as my ex is such a slippery arse but I’m not sure what to say to the solicitor.