How can I make my housemate pay his share of the bills?

Ok so I moved into a new house in September and it is only me and one other guy living together, we both go to uni together and only met through a house hunting event held by the university to help people find housemates when they don’t know anyone.
We talked about bills and as we both receive funding we have a joint account we both pay into every month to pay bills. I sorted out the internet, phone, tv licence and he was supposed to sort out gas and electric and water however I have had to sort that out now as he didn’t bother. We are only in November and he is already behind on the bills, he hasn’t even fully paid the first month’s bills and so I am having to cough up the cash so that we don’t get fined or cut off etc. I have been going without to make sure we pay everything ok however now he has been spending about £40/week on cannabis. It’s not him smoking that bothers me at all and I definitely wouldn’t want to get police or anything involved but how on earth is this more important than bills? I have asked him many times to pay the money and he keeps telling me ‘next time his money comes in’ i.e. his maintenance grant, but I know this will mean that he will just be short for the months afterwards… There is no way that I can move out or anything and I really don’t want to cause an argument over this right now as we both have a lot of work on and I know we are both stressed out. I just feel that I’m being taken for granted here as we are in the same position financially.