Does it start to irritate him if you play around with it?
Is it hard, or squishy?
Take him to a vet tomorow to get a biopsy done on it, to make sure that it's not cancerous.
My flat coated retriever has a lot of fatty deposits that are SOFT lumps on his body. They don't bother him, as they're in areas like his back or his chest and aren't restricting movement.
He has also had benine tumors - that were both in areas that bothered him, and areas that don't. A lot of dogs to get these. If they are in an area that it doesn't bother him, we leave them. If they're in an area that DOES bother him (like he got one in the pads of his feet, inbetween two toes) we get them removed.
But you always want to get a biopsy done on them to make sure that it's, like I said, not cancer. I know, I know -the big C word - it can be scary, but it's better to know NOW then take your dog to the vet later and find out there's nothing you can do about it.
If it is cancery they will do x rays and CT's to make sure there isn't any bone loss and to see if it has spread. If it hasn't, they'll remove the lump and you'll go in for regular checkups.
Most often though it is a fatty deposit or a benine tumour. Dogs who have cancer (that has spread, espcecially) tend to be more lethargic, or have decreased appetite. They just don't act completely themselves. So if it's just a lump that's making it uncomfortable to walk because of where it is - it's probably one of the other two
Good luck, and let us know what the biopsy shows
meghan
St. Bernard limping 
St. bernard 
