Hmmm, just read your sleep post as well, seems like your Thursday afternoons are a whole lot of stress!
Does your MIL try and feed him when he isn't hungry enough/too hungry?
Does your son eat finger foods, could MIL have finger foods ready the minute he's in his highchair for him to start munching on whilst she heats up the rest?
Is Riley uncomfortable in the highchair? Could you strap him in his pram and put a big towel over his knees and the pram (to try and prevent too much mess) and feed him there?
Could MIL & FIL eat at the same time as Riley? Even if they only had a small snack it might make Riley happier knowing he's not eating on his own.
Does he need a bit of wind down time before mealtime to get him calm enough to enjoy his food, maybe his Grandad gets a little too excited!
I've not any experience of this type of problem (my problem is that Dom goes to my sisters where he happily eats far too many sweets and crisps...I tell myself it's only once a week...) but hope the ideas might help
Sal
x