I am not sure if it is different because it is your Son, but when a friend was on benefits, her partner was only allowed to stay three nights per week before he was classed as living there and have to pay rent.
the three nights thjing is a myth, at least in the area i live in. My local hiousing office say if someone stays "regularly" they have to reduce you benefit. Its the regularly thats open to interpretation, and could mean 1 night a month - why dont you ring up, dont have to give your name and ask?