Yes, proper nappy if you can manage to get one on him now. The rest seem to hold much less volume.
I've heard people suggest a cloth nappy booster inside a disposable nappy to increase absorption?
What colour juice? Black and red is supposed to irrigate the bladder, so many switch to orange, or water/milk?
Also, can you try to get him drinking larger volumes earlier in the day, so his bladder stretches slightly, they he will be able to hold more, and maybe survive til morning?
DS2 had to go back into under poppered vests, as he had his hand down his nappy, causing leakage. At 4 he would be too big for this, but is he fiddling, resoling in wee escaping before being absorbed?
For the sheets - get a decent waterproof layer, and then double make his bed - so waterproof sheet, sheet, waterproof sheet, sheet. Then in the middle of the night you only need to strip off the top layer, not put new sheets on. We also had a spare duvet, with a cover on that was ready for nights when duvets got soaked as well....