If your son will be acting as trustee for you then he cannot get the grant.
If he is really buying in his own right then you would have to be on title to get on the loan, but he would also be on the loan too. This would also prevent him getting the grant. I don't think you will find a lender who would let you take the loan without being on title.
If he gets the loan in his own name with help from you, may one of those family guarantee loans then it may be possible to have him only on the loan and get the grant.
It may also be possible for him to buy in his name and you do lend money to him (eg from LOC) for the purchase and have the property unencumbered.