lHe's not allowed according to the rules, he's down on record as saying it's completely daft, he has a flat and he wants to stay in it but after the expense scandal, that's that. He bought it to stay in while you still could claim mortgage payments, it's not as if he could just nip home when he wanted to, so it made sense at the time to buy.
Of course, he could stay in it and just pay the mortgage, but even I would consider it daft if I was no longer getting support for that and free hotel accommodation.