I couldn't find anything on synergies stacking, but I did find the following (PHB, pg. 66) "Your DM may limit certain synergies if desired, or he may add more synergies for specific situations."
Other than that little caveat, looks like your player did a great job finding all the bonuses he could.
Personally, I would limit him to one synergy bonus according to the situation. It is unlikely that both a Bluff and Sense Motive Synergy would work at the same time. And when you are dealing with peasants about to revolt, Knowledge (Nobility) doesn't do much.
Just my 1/4 cent.
Other than that little caveat, looks like your player did a great job finding all the bonuses he could.
Personally, I would limit him to one synergy bonus according to the situation. It is unlikely that both a Bluff and Sense Motive Synergy would work at the same time. And when you are dealing with peasants about to revolt, Knowledge (Nobility) doesn't do much.
Just my 1/4 cent.