I have a 20 month old male weimaraner. Who has been castrated. Recently when
playing with other dogs he has been protective of toys or sticks. but this only happens with certain dogs. Can anyone recommend a way of stopping his dominance.

Resource guarding is the most natural thing in the world between dogs. After all, in the wild, he who has the most resources wins :D

Dogs do not "DO" sharing!  If you are unhappy with this behaviour I would strongly suggest that play with other dogs is restricted to the sort that does not involve toys (dogs should never play with sticks, they are far too dangerous and can cause serious injury if not death) and/or restrict him to playing with dogs that are not as competitive.

http://www.colliecorner.com/stick-to-toys/home.htm

It is not "dominance" but resource guarding.  Resource guarding is really only an issue if the dog exhibits the same behaviour with humans. 

HTH