I’ve had this problem a few times previously, except I sit on my own and had two large fellows sat either side of me. They both encroached on my seat and I had to squeeze into half of the usual space. The worst bit was the guy on the right who always had his arm in front of mine, effectively pinning me into my seat. I wouldn’t have minded so much if my arms could have been the ones in front and I felt very trapped at times. I started getting there late so he was already sat down when I arrived and I could sit down last. Practical suggestions: 1) move so you’re sat next to him. Your lad won’t be as squashed and the guy might not ‘spread’ as much into you 2) swap places with your wife. Rather than your son leaning into you and you leaning into your wife, there might be a bit more room with your son and wife sat next to each other and you can have a full seat in peace
As daft as it sounds that's Actually probably the best idea. There is nearly allus an empty seat at side of me. And the largest one of us parks our bum on both. In the middle. My lad and his 2 mates have 4 kids between em as season ticket holders and it's rare for all 7 to be together at one game. So spread out.
Just speak to the chap, generally find communicating with people as long as it's polite and done in the right manner gets things resolved, he's probably oblivious to the situation or maybe thinking the same. Another option if your not comfortable speaking to him would be get in your seats early and set your stall out, e.g in a way that son is comfy and let him take the hint.
Say to him, as Blur once said, "You should cut down on your pork life mate, Get some exercise" Although I always thought they said cut down on your pork pies!