I may be wrong, infact I probably am but anyway. When we signed our contract I questioned what would happen if one of us didnt sign and she said because all the tenant's weren't on the form it was void, however that therefore meant we didnt get the house as we weren't all agreeing to the conditions. So therefore its possible you could argue that and look for another house. However although it is his job to pay, technically all the liability will fall to you three until a new person moves in. Unless this Tom is legit and pays.
Another thing is a fall back payer, someone if Tom couldnt/ wasnt paying would pay, generally a parent, but I guess he never sorted that out?