If he's running away from you, then you need to go backward in his training. He's afraid and not trusting you to keep him safe. I would work him in the halter on just the basics for a little while longer. Walk him from both sides all around the property and out on the trail. Let him eat some, work on him respecting your space, and just general ground manners. Be CONSISTENT in how you want him to act. Let him investigate scary things, let him graze when he's being good, and just bond with him.
Once he's trusting you more, start sacking him out. First just use a small towel fresh from home. Rub him all over with it, put it over his head, twirl it around above your head and to the side, gently slap it on his shoulders, neck, chest, back, belly, legs, and rump. Just keep at it until he's calm. Give him some treats and quit for the day. Rinse and repeat (with a fresh towel each time) until he doesn't care anymore.
Then move to a bigger saddle blanket (30x60 or close). Flap it around, shake it, rub it on him, flap it on his shoulders, chest, back, etc. Put it over his head, make him walk over it, and just generally torture him with it until he doesn't care what you do with it. Always end on a good note with him calm, give him pats/praise and a treat, and quit for the day.
After he's good with the balnket, then bring out the saddle. First just hold it so he can sniff it and check it out. Have someone help you if you can't hold him and the saddle at the same time. Hold it next to his shoulder, rub it on him, go around to the other side and do the same. Rub it on his belly and hips. Just keep at it until he's calm. If he was scared of it for a while, then quit there and keep trying the next day. If he did well, then go ahead and put it on his back. Hold it and walk with him if he moves away. Just stick with him until he relaxes. Pet him, give him a treat, then take the saddle off and quit for the day. Rinse and repeat until he doesn't care about the saddle, then cinch it up! Next step would be the pad and the saddle.
Anyhow, that's my method for getting a horse used to the saddle. You definitely don't need a round pen