Is there any heat on his legs? Have the lumps changed before? Are they changing now since the competition? Off hand, I'm thinking of tendon strain/injury, especially if he had lots of time off, then worked hard right into jumping. Also, if there was a previous injury (that you don't know about, sounds like) to the area that caused the lumps 'way back when, of course the area is more susceptible to future injury, causing more/bigger lumps. Seems to me that bone damage wouldn't be lumpy like that?? I dunno.
If there isn't heat and no pain, the only other thing I can think of is windgalls, but seems unlikely.
If you are worried, have a vet come out and let him decide if an ultrasound or an x-ray will tell you more. You won't know how to help if you don't know what it is. In the meantime, if there is heat, some ice tight twice a day with cold hosing will carry you through.