about 2 weeks ago I got him fixed (neutered), I live in Los Angeles and they are strict on new laws about unfixed dogs etc.
My concern is since this past weekend Duke (my BT) has been showing a lack of interest in training and playing.
Last Friday my wife brought home a friends dog (12 month Beagle) to baby sit for a few hours, they played for 3+ hours,
Saturday showed no signs of energy nor wanted to walk on his leash, he'd rather stay in his bed and do nothing.
Saturday evening and sunday afternoon played with my brother-in-laws dogs for a few hours.
Yesterday, Monday slept all day and I had to carry him out to the front door to take him for his walks, he's also been having a little bit more drooling or salivation.
Prior to the weekend he was suffering with some diarrhea and was on medication for 3 days. Anti Diarrhea meds has stopped as of sunday.
I also started him on a RAW Diet this past weekend as well.

Is there something That I should be alarm of or is it from this past weekends activities that has taken a lot of him?
He still eats, shows signs of wanting to play but doesn't seem to have the energy.
Today I have an appointment with his vet.
Any thing from your end would be of great insight and help.

it could be the excerise combinated with the changing of food,medicine for the diarea and the putting to sleep with the operation. this takes a couple of week before it's out of their system. but it's always wise to ask the vet!

It sounds like a combination of things going on with little Duke.

First of all he had MAJOR surgery and was under general anesthesia which is very hard on a body.

Second of all, stress, loose stools, meds, excessive exersize, etc.

Keep him on the raw diet, let him rest and just regular exercise for a while, do let him over do it.

If his appitite is not good then he may have some inflammation going on and his body is busy taking care of it, this would cause more legeragy as well.

How is he doing now, since you first posted?