I would try not to request money from his loan, but the bursary is presumably to help cover expenses due to your low income. At least some of this should go towards travel (train and your petrol) and some board money to you.
It is expensive supporting a grown adult in the home, especially now you will have lost things like Child Benefit and any associated UC top-ups, free school meals, etc.
My son is living at home for his final fourth year and he gets a bursary from my DH's employer.
He kept the money the previous years, and used it instead of getting a student loan for maintenance. (They also paid his tuition fees, so he was lucky to not need any loans at all.)
My son said he did nothing to earn that money, so we should keep it this year as we are housing, feeding and supporting him. We also pay his train travel and drive to/collect from station half an hour away. (Rural, no feasible public transport nearby.)
Don't feel bad about asking your son to cover at least some of his expenses from the bursary. It is a shame he hasn't offered, but 18 year olds generally don't appreciate how much they are costing you.