Should i get back together with my childs father?

My ex and I were together for 3 years before I broke up with him... We had a beautiful baby boy together, who just turned three. We have been separated for a year and a half and I miss him greatly... However, the reasons we broke up are still valid...

He smokes weed all the time (I don't smoke or allow it around my child), he works a dead end job and has no career in his future. He is also a very negative person and always makes a situation worse by being negative...

Now that being said, I still love and miss him. We both just recently left the people we were worth and I'm wondering if we should go for round two... The reason I am cautious about this is because we have a really good relationship as exs. We eat dinner every other week as a family and have shared custody of our son. But I miss him... He knows this and he misses me but he says he just left the girl he was with and is not ready.