As you don't know what will happen in your children's future divide everything equally. Nobody knows what the future holds and we have seen only too recently how things can come to a halt and crash.
With the share of money your son would receive, it would hopefully be enough to put a down payment on a studio or something bigger because he is saving money while he is living with you. If your other children don't need their part of the inheritance at moment, they can decide that their share will help their children onto the housing ladder when the time comes but that is their decision.
Moreover, who knows that maybe your son falls, as many men have before him, for a gold digger.
Family does not share money, as several friends of mine have found out to their cost.
Recommendations please, for a stopover on the way to Loch Tay