Roommate doesn't want to finish lease

[Patrick Spencer]

I live in Queens, New York. I share a two bed apartment with my roommate. Recently we found mold on his walls. I helped him clean it up and we informed the landlord. The landlord told us the excess moisture was caused by my roommate fiddling with the radiator and causing steam to leak. 

My roommate signed a sublease with me until the end of February. He says he is moving out three months early and he is not going to pay the remaining rent because of the mold and because he is cold at night. Earlier, before all this, he also told me earlier that he got a new job in New Jersey and he can't afford two rents. I can see two sides to this: one, he wants to move and can't afford two rents so he is finding an excuse; two, the room actually did have mold which isn't habitable and it is actually cold at night (around 60 degrees). 

I told him that he can't legally break the lease but he says he is going to do it anyway. I have a one month security deposit I am going to keep if he leaves. Not sure what to do. Am I being unreasonable to think he shouldn't break the lease? Thanks.