Hello, I am happy to answer your question.
This is a pretty typical situation with roomates. In my opinion, it is better to have just one person take full responsibility for the lease so that the others can merely "sublet" from them and can leave at anytime, but the person on the lease always has the right to stay.
Since that is not your case, you will have to go with the flow. When you sign your name on the lease, you are taking responsibility for the entire amount of the lease, even if there are two other people on it. Thus, if the rent isn't paid, the landlord can go after EACH of you individually for the entire amount due, UNTIL he gets paid in full.
So, you have taken a huge risk that if the others decide to bail, you are responsible for their shares. This is why many landlords don't take shares, because they know that if someone leaves, the rent doesn't get paid.
Now you need to figure out what steps to take. First, make sure you have enough money to keep paying the rent, even if this person stops paying and moves out. Next, ask them if they are willing to be released from the lease so that you can get someone else in there ASAP. If the landlord won't change the lease, ask your roomate to sign something saying that he agrees to allow someone to take his space in the apartment because he can not pay. You must get tenants in there immediately if you can not afford this alone.
The others will remain liable on the lease, but this way you can all move forward and still pay your rent without paying an extra share each. Then just take this as experience to learn from and next time make sure to only have the lease in one person's name.
I hope this was helpful and good luck.