I think one night a week is reasonable (I'd say to him pick one or the other). It's important for him to have an outlet to relieve stress and keep fit if he wants. As do you! I'd think the footy is better because its fitness like the boxing but also social/team stuff. Also it's setting a good example for your son about team work and exercise. Might get your son interested in doing kids footy later if u take him along to watch his dad play on the weekends sometimes.
You should think of something u can do for yourself once a week or fortnight and have him look after the kids during that time. Even if its having a regular walk.