(2-part post):
Before you act on the following it would be wisest to remember the old saying that "you can't get blood out of a stone". If he's really broke (and with virtually no 'disposable income' for the foreseeable future) you might be wasting your time.
If someone owes you money the only way of forcing them to pay up (which doesn't involve a couple of big guys with baseball bats) is to threaten legal action and then, if necessary, to take such action.
Step 1: Send your housemate a formal written demand for the money. (It's extremely unlikely that a court would consider making an order against your housemate if you've not done this). The demand should be dated and clearly show both your name and address and that of your housemate. (I know that seems a bit pointless when you live at the same address but you're trying to follow the procedures which the courts expect). It should also state the amount that you're demanding, and the reason why you're demanding it. Further, the letter should state that unless full payment is received within a reasonable and clearly defined period (perhaps 14 days?) you will commence legal action.
You must keep a copy of the letter for your records (and to produce if a court requires it). Normally you should send the demand by recorded delivery but, since you share the same address, it might be better to ask your housemate to sign and date your copy to prove that he's received it.