I am concerned about the size of the lump and that it is very hard.
If it was soft I would say they have knocked them self and it is just a hematoma, I would monitor it and see how it goes for a bit.
But being very hard and to have come up so sudden is strange. Personally I would monitor it for a day or two and see if it changes in texture and size, even measure it so you accurately know if it has got bigger or smaller.
Feel to see if there is a bite site, a small lump that may have caused the overall area. Feel for the texture of the skin under the hair, like rough flaky skin under the hair to show irritation.
If he has been laying down a lot in his stable then it "could" be from his hoof hitting the area. But I would more think it to also have some hair rubbed off or other marks on the spot if this were the case.
A girl at our riding club got a similar size hardish lump a bit higher up on the side if the horses tummy and it resolved it's self in 2 weeks with no treatment.
I went out to find my TB gelding cut his hind leg. I would say he did it late evening after feeding and by the time I walked out yesterday morning his whole lower leg was swollen from his hock down, looks like a balloon.
It is the smallest cut you can imagine in the middle of the leg and it is hard to imagine it has caused so much trouble. Despite my husband and I walking the paddock we can't find what he has done it on!
The leg is warm, yet he is walking perfectly normal. As I walked out I could not even tell he was injured from a distance. I am treating it myself for the time being because I have the required medicines and knowledge to do so. But if he turns I will call the vet immediately, I refuse to take risks with my horses.
In any case you have established it is not tender or hot. I would call the vet tomorrow after checking it again. If your really unsure give them a phone call now and have a chat. They will answer any question for free and from your description they will be happy to book it in for a day or 2.
BTW - that is one clean stable with lovely fluffy bedding! It is nice to see horses kept in such lovely clean conditions.
I hope the lump turns out to be nothing, good luck ;-)
ADD - Oh Donald why are you going out of your way to be so nasty? Your "bio" the other day stated you were a knowledgeable TB horse figure with may years experience.
So I can only imagine you have become bitter and twisted or your not the knowledgeable person you claim to be? We have many people lie on here and claim to have much more experience than they do.
They are the people that leave rude answers because they don't have the experience to back it up. Such a shame, I was happy to think we had gained a valuable Y!A member ;-(