How old is DGS. Children can be easily swayed. Perhaps he really did think he wanted to spend time alone with his cousin - at that moment in time.
Would it be an idea for you do a bit of gentle probing next time you see him and try to find out what the lad himself really wants? He may genuinely have gone off football! (Unlikely, but he may have!). If he does still want to go maybe DiL could take him to training for a while. It seems a shame for him to miss out and I'm sure she'd soon get bored with it!
Lest we forget what we have just lost ....



and
or if you need something stronger 

