Whatever it is you do now to get him to lay down is what you do for the first step of the gradual withdrawal. I assume at some point he lays down and calms and then goes to sleep (he is not sleeping propped standing up is he?). That is when you take your first step. The first step might be to say a key phrase "Have a nice sleep, call if you need me" and sit half a meter away from the cot - you then stay there until he is asleep. The next step on your plan takes place a couple of days later and might be to sit a full meter away from the cot so you would do whatever you usually do to get him to lay down, and calm, say your key phrase and move to the chair a meter away and sit there until he is asleep. If during this sitting he stands again or cries because you are not right next to him then you go to him calm him, get him to lay down in your usual way, say your phrase and return to the chair. A couple of days later you move the chair another half meter away and repeat the process, fully calm and laying down, key phrase, go to the chair, return if needed.
Yes it may take longer to get him to go to sleep for naps but that's sleep training. Ultimately he will sleep better because he will have the confidence to self settle rather than relying on you to be there. Plus once he is self settling my bet is he has a more relaxed Mummy, which is always a great thing for LO.
