Just properly engaging from the hind end and lightly carrying a headset on his own, will, overtime, build his topline.
Just start at the walk, and start working on squeezing him forward into the bridle. When he gives to the bit release him. When he looses the headset, squeeze him into it again, when he gives release. Just keep doing that, until he's comfortable traveling in a soft headset. Slowy build it at the trot, then canter.
This will take some time. He hasn't used those muscles like that in a long time, so don't overdue it for the sake of soreness.
"You're just as sane as I am."~Luna Lovegood.