My 19 month old GSD is a lovely boy with people and other dogs.  He will happily play with other dogs and shows no signs of aggression.  We are considering neutering him for various reasons but I worry that he will become dog aggressive due to the change of hormones.  I have seen other dogs have this happen including one of my own previously where they have become fear aggressive (I think) with other dogs post spay.

Has anyone had any experience with this or can simply tell me that I am worrying too much.  Also any advice on making sure that he stays his happy self with other dogs post spay would be appreciated!

What are your various reasons for castrating him?  I only ask because if he is a friendly well mannered dog then staying intact won't do him any harm.
My mums male GSD is 6 and has been great so no need to castrate, she is however considering getting him done asap due to other bitches at home and the commotion he causes when they are in season.

but I worry that he will become dog aggressive due to the change of hormones

He shouldn't provided he is fully matured (mentally and physically) when he is being neutered.
I have loads of neutered dogs running around my place of either gender and have never found
the de-sexing to make a difference in their inter-dog-behaviour.
The only reservation I would have with regards to de-sexing a dog is when they show juvenile
dog aggressive behaviour as I find de-sexing them too soon will enhance the aggression.

How would you describe juvenile dog aggressive behaviour?  He wil occasionally bark/growl at another dog (semi in play) which we are working through (only when in close proximity, on lead and when they are being pushy with him).

We are thinking of neutering him as he is a working dog (obedience and agility) and we are now struggling more and more with him losing focus during work, going off 'bitching' and being very interested in other dogs (going up sniffing etc).