And yes, he is worried about mares and paces.
There is your answer. I'm afraid that as long as he worries himself like this, he will not maintain any weight no matter what you feed him because he will just pace it right back off.
You can try a few things to calm him down such as put him where he can see the mares.
Put him out of the sight of the mares.
This is reaching but you can try some toys, or one of those "Bizzy Balls" that are like a big treat.
But if he were my horse and this upset, I would put him on a training schedule and get his mind off of himself. I'd keep this guy busy with ME. Even if I didn't need him trained for anything, I'd be round penning him, teaching him to sidepass etc. go over obstacles every day. He'd be tired, and then I would do a really long cool down afterwards and then sit and vege with him.. Then I'd be washing him up using some Absorbine Refershermint and make him like a brand new little man. Then, the ultimate reward would be his grain and hay. This should slow him up a bit and give him time to get some meat on his bones.
I find that horses that have too much time on their hands, and are bored or like to worry, tend to be happier when they have a JOB. When they are forced to use their minds and keep thier minds on you instead of the mares.