Welcome to YT!! Glad to have you both here! What's your guy's name?
Is he really 2 yrs old or was that a typo? I'm just confused, because some of the other things you said, made me think he was a new puppy.
I'm not familiar with litter training, but if you use the search option on here, you should find some good info.
I'm wondering how much you've changed his food from what he was eating before? What type of dog food are you feeding. You could very well be right that all the delay in poo comes from a change of environment.