Likely it's not that he can't clean himself off properly that's an issue, but that his poop is mushy and so sticking to his fur. Most often when there is mushy poop, but the fecal balls are normal, the mushy poop is due to improperly formed cecotropes from an imbalance in the diet. Usually too many carbs and not enough fiber.
There are a few other possibilities as well, like if the cecotropes aren't mushy but are properly formed and sticking in his bum fur. Then it could be that being a lionhead, the fur is too long and catching onto the poop before he can injest them. Or being overweight or having arthritis can make it difficult for a rabbit to reach down and ingest the cecals. So they may instead, get stepped on and smooshed into the fur around the hind end.
So you'll need to evaluate which might be going on. If he's having difficulty ingesting his cecals when they come out, or if they're coming out mushy and getting stuck in the fur.
https://wabbitwiki.com/wiki/Rabbit_poop
https://veterinarypartner.vin.com/default.aspx?pid=19239&id=4952892
What's his diet - type/brand of pellets and hay, any veg, treats, supplements, meds? Have you ever tried making any changes to his diet to see if it helped clear up the mushy poop (if that's what he has), primarily increasing hay and decreasing other foods?
Just FYI regarding the flea collar, or any kind of collar. They should never be used on rabbits. Rabbits aren't sensible like cats. If a rabbit gets the collar snagged on something, they can freak out and end up strangling themselves. Along with that, the chemicals used in a flea collar may not be safe for rabbits.
There are only a few flea treatments considered safe to use on rabbits. The regular Advantage (which I'm not sure they sell anymore) and Revolution (or other selamectin flea treatment), with Revolution being the better of the two (in my opinion). If you need to give your rabbit a flea treatment, Revolution (selamectin) is what I'd use. Just
never use Fipronil on your rabbit as it's toxic to them. Or on your cat if your rabbit and cat live together and your rabbit could groom it off your cat.
https://wabbitwiki.com/wiki/Fleas