I can't help with an a/c I'm afraid, i dont have one, but it's worth contacting your local council to see what voluntary support groups are in your area. Even if they can't help directly, they're generally really well connected and could signpost you to someone who can. (Eg your local Lions group)
I know you said you don't have loads of room in your freezer, but if you can fit it then half fill a hot water bottle and put it in in the morning, it should be frozen by bedtime. Wrap it in a towel and it should help keep your nan much cooler. If you have 2 on the go, you can use one during the hottest part of the day as well.
Just make sure you buy a new one before you use it as hot water bottle again! Or you could freeze damp face flannels for use during the day.
Do you have a bath that your nan and son will be able to get in and out of? Cool baths can help. Also tell the discharge team that you're worried about your sons recovery in the heatwave.
Ice lollies help enormously- the Mr Freeze ones are cheap and you just freeze a few at a time so they don't take up too much room.
Jelly drops are expensive so I appreciate they might not be affordable for you, but they're a great way of getting fluids and electrolytes into elderly people.
Make some ordinary jelly too.
You could speak to a pharmacist or gp too about whether it's safe to give your nan electrolyte rehydration drinks like diarolyte.
Something they do in hot countries, although I haven't tried it myself, is to hang a damp sheet on the inside of an open window which might be idea at night.
I wish I could help more!