Depression or Something Else?

I just put Kyiro and Lily in the car and took them over to Kyiro's favourite place to cheer him up with a short walk there. We only walked for about 10-15 mins, but the whole time Kyiro showed no enthusiasm and just dragged behind on his lead with his head down. Not even a squirrell hopping around got his interest.
He has been like this on his little 5 min walks I have given him recently.
Bearing in mind he hadn't been for a walk for nearly 3 months, I would have assumed he would be excited to get out again.
He wasn't really limping so I can't put it down to the fact it was causing him a lot of discomfort. I have seen dogs out limping badly still enjoying their walk.
Kyiro is only 6years old and I have had him since he was 3yrs and he has always set off on his walks trotting happily beside me but then after about 10 mins does start to lag behind.
I'm not convinced this lack of enthusiasm is connected to his foot (although it may be). I wondered if he really had sunk into the depths of depression or if something else is going on which has always made him a lethargic dog, which has got worse.
The only thing that makes me think its not depression is the fact a couple of days ago he picked up a toy and played for about 2 mins and he has never played with toys before. Surely he wouldn't suddenly play if he was that depressed.
I am wondering whether to mention to my vet on Monday?, although I must admit I really do not want to believe he could possibly have something else wrong.
Just after this walk just now its bought it home to me with a bang that he is not happy with something.

His walks have been painful for quite some time so my guess is that he could be anticipating pain as well as actually experiencing some so is less than enthusiastic about his walk at the moment because of it.

Hopefully, he'll soon be sorted out and can get back to enjoying himself again