Thank you for your question
My name is Clare
I shall do my best to help you but I need some further information first.
How old are his siblings and how much is involved?
In fact your son is under a duty to keep the money safe until they are each 18 years old - indeed he would be personally liable to them if the money disappeared.
With Interest Rates so low he could consider purchasing Premium Bonds in their names - and offer any wins to the mother to use for gifts.
I hope that this is of assistance - please ask if you need further details
Yes he can