I've been cooking for years but I've never been in a situation like this- but I'll try my best to give advice because that's what this website is about.. amateurs giving advice to other amateurs! (Im jkjk) Anyways, maybe just think about the reason your grandpa is so insistent on cooking food for your family! Maybe he really wants to help and feed you guys, even if he may have taken it a tad too far. Maybe it's his everlasting love for you guys that drives him to becoming Gordon ramsey! (Ok i get it it's not funny) Now that that's cleared up, if your grandpa *knows* that his cooking kinda sucks and is refusing to let anyone help, maybe it's his ambition or ego that's not backing down.. In that case, maybe try to cook him really good food and make him realize that others can help him make better food? If he thinks that his cooking is actually really good, then get friends or other people that he is close with and get them to nicely tell him the truth! I wish you (+family+health) luck.