Loss of focus during training sessions after starting walks

My puppy is four months old, we started the walks almost a week ago. I decided that our next command would be "heel", but I noticed that his attention span became almost non-existent. Obviously I don't want to overload him, so I've been doing a session every morning after his first outing, but it still lasts like, two minutes being optimistic before he decides he doesn't want to do it anymore, when before we could do five or seven minutes focused twice a day.

I guess it's normal, he gets several treats and some outside commands, as well as leash training and potty training, so I'm not going to insist on more sessions or longer sessions since again, the last thing I want is to overload him.

My question is will this last long? What is the best time to do that single training session and obtain the best results? Will he show interest again when he settles into the routine or do I have to permanently change my approach from now on? Maybe offering you high-value treats will help? I haven't done it because I don't want them to lose courage, I'm only giving them to them when they go to the bathroom outside.

Any guidance on how to go about this from now on to keep it fun for him would be appreciated, even if the advice is not to teach him anything new at all until he settles in or something, I'm totally willing to wait, I just don't really know what's best.