Try arnica and witch hazel, both have bruise reducing properties. TBH you don't really need to do anything with bruises as they go all by themselves...
My foal had one, he didnt take too kindly to the hose but we held sponges of cold water on it to help but as Inky_and_Sunny said theres not much you can do, it'll go away by itself.
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Arnica tablets - give them in a piece of apple or carrot - he won't even know he has had them. Helps with healing the bruising and healing of soft tissue and speeds it up. Get the tablets in boots - the 30c are the best.
Just remember that at the end of the day a haematoma is a bruise. If we get one what do we do? Why do we worry so much about our horses - because the vets give them big names!
As others have said, it's a bruise - but some can be huge and so filled with old dead blood that they hang downwards like a water filled balloon. Gentle massage can help - it's only the gradual re-absorption of the old blood cells that will get rid of it so the best thing for a small to medium one is massage, movement and time. For big bloated ones, I think the vet can lance it to speed things up if it's eg in an awkward place.
It depends on how bad it is. Cold hosing and ice is good. I've just had a massive one on my leg, and was told to buy a cream called Hirudoid over the counter, which was WAY better than arnica. I would expect it could be used on equines, but perhaps check. As BoxofFrogs said, it can clog up with old dry blood sometimes - it goes like chewing gum basically inside. My physio said this needs to be gently massaged to break it down - but obviously not too hard, and not while the bruise is brand new...
I hot compress them.. followed by cold hosing. It works quite well. If its a big haematoma (like someone said above if its like a balloon) it may need draining.. but other than that.. i'm just repeating what's already been said.. its just a bruise!