Rebekah, have you had him checked out by a Dr to rule out something like ear infection, if Aiden wakes lots 2 nights in a row I normally take him off to the Dr & he has on occasion had an ear infection.
What we do is either sleep on the floor with him for part of the night or in bed with himnow he is in a single bed, as to me he has a need for my comfort... I don't have the expectation that my ds's will sleep through the night every night, so I guess I just to a degree think of the nights I get unbroken sleep as a big bonus, but that is just me, so if he wakes up & needs me (bad dream teething, sickness) I comfort him the best way I can.
I don't mean to undermine you here, but if you have left him to cry (but answer your babies cry's) he is probably very confused & saying, well if I cry more & louder Mummy will help me eventually... so I think for a while you have to answer his needs & explain you are there for him too when he crys. If you keep doing it- like when you were in the same room, he should go back to "trusting" you will meet his needs.
Does that make sense