Hi .. I am in a very similar situation.
My guy was in a mono relationship when we met - he was very unhappy and has ended it with her.
He is poly (well, he wants to be) but so far we have only discussed having another woman with us rather than separate relationships altogether .. we are in a LDR which complicates things even further! (he was/is planning on moving here but he just dropped a bombshell on me a few nights ago that this could tak up to 12 months + I am still considering whether I am up for that).
I want him to be happy but am taking it slowly & I don't think I oculd ever have another relationship concurrently although I would be open to FWB. I am just not wired to feel romantic love for more than one person at a time (I don't think) but, I can certainly feel lust.
I guess you're guy needs to talk to his current GF and see how she feels about addressing his needs .. my guy tried this and it just didn't work - the relationship went to hell soon after that (although it was unhealthy t begin with) .. that seems like it would be step 1?
"Well, you know, I have a choice. I can spend my life waiting for Xander to go out with every other girl in the world until he notices me. Or I can just get on with my life."
"Good for you."
"Well, I didn't choose yet." Willow and Buffy