Welcome Momrat!
If MoMo is ok with grooming himself, let him clean up himself as best possible. Then you can try comb out anything or wipe with a damp paper towel. When I first took my rabbit in the car, she got some messy poop stuck on her and I tried wipe it off. Turned out, she cleaned herself up better than I could. If he’s making a mess though, you may have to get in scoop him up and secure him firmly in your grasp to your body. It’s important to support the hind end, so maybe get someone to help with the cleaning part. I find sometimes when they’re on the ground, I can put my hand just behind the head and gently push down. They stay still, then I can scoop them up into my arms.
Has this started since you’ve given him the new feed? It does sound like it may be too rich for him and he’s producing excess cecals. Usually they reingest these softer poos so you don’t see them lying about – only when they’re over producing them. Does the pellet mix have seeds and grains in it? Adult buns do better on limited servings of a plain timothy based pellet, younger ones on an alfalfa based one.