Either Verulam or Jo could be correct. The smaller and now aggressive dog could be INJURED; so see the vet to rule that out. It is NEVER advised to have or own two dogs that are more than 25 lbs apart in weight due to the HIGH LIKELIHOOD of injury - to the smaller dog during play.
Secondly, he may INDEED be objecting to loosing his place in the pack hierarchy of the household, as the Lab matures and takes over. That is not something you can interfere with or fix, other than to prevent a bad fight from breaking out - since YOU (NOT they) should be IN CHARGE of the home (at all times).
Hard play may no longer BE appropriate, either due to fights or due to the likelihood of injury. MAKE IT => SO! If either or both dogs have not had obedience training, get them in classes, so you gain some control.