You poor thing, you have such difficult medical problems, I'm not surprised that you are struggling, but like some of the other posters, I believe his behaviour, even for a 15 month old baby is totally unacceptable, IMHO, it's totally unnecessary for any c child of any age to destroy your home as he is, and, at his age he needs to be taught this.
Children of any age need boundaries, continuity, and clarity, along with love. It doesn't have to be under the same terms as his parents, when with you, he needs to learn YOUR rules?
I'm not sure if you have him all day every day, but you need to almost timetable his activities, if you can, buy a mini set of cleaning kit for him, quick cuddle with him, then he can help you do some jobs, most chiLdren's will enjoy this. If he becomes aggressive tell him he is begin naughty then tell him to be a good boy and help grandma. When he helps you give him lots of praise.
Next is break time......a piece of fruit and a drink of water or milk in his high chair, for him, and a cuppa and fruit or similar for you. If he throws them tell him 'bad Freddie' and remove the items. You can remain sat down, and enjoy your drink and fruit, but don't give Freddie his back.
On to the next time, play a game together, keep telling him what a lovely good boy he is. If he misbehave tell him, calmly, and quietly.
On to lunch, ONLY give him healthy food, praise him when he eats and ignore anything pushed / thrown away.
If you are up to it, go out for a walk, feed the ducks, make the leaves crunch. On the way home, ignore him, pop him in his buggy, he should sleep. This is your rest time as well.
Now it's time for dirty play, craft, mud pies, whatever you fancy......are you getting the idea?
It will be he'll for a few days, but I can almost guarantee that it will work, and will be worth it!