I don't have a soothing method
Then it's time to get one
I know you have an angel baby who tends to coo and smile before sleep and coo again when he wakes, playing in his cot too until you get to him. The thing is through his baby months, his toddler years and his entire childhood (and let's face it, his adult hood too) you will be called upon to sooth and support him, that's what we do, when there is teething, a bumped head, a cold virus, when some other toddler pushes him over or takes his toy you are the one who will ease his upset. You are already a great source of comfort and love to him, you are his world, but perhaps you still feel unsure how to help him when he calls.
So let's find something you are comfortable with.
Think about times now when he gets upset, what do you do? Pick him up, speak to him, rub his back, hold his hand? These can all be the basis of your soothing method so if something comes to mind let me know and we can incorporate that in to your sleep soothing method. OK?
A simple and effective idea is that you produce a mantra of your own, a key phrase, something that is repeated over and over in a pleasant and reassuring voice, eg "Everything is ok. I'm here. Go to sleep." When you repeat this he hears you, and he can focus on this to help him calm and feel drowsy.
Along with this you might put a firm hand on him so he can feel you. You might pat, rub or slightly rock with this hand so he can feel you.
What do you think?
WRT the mantra cry. Some babies do mantra before sleep, it is a repetitive sound which does not escalate. Some babies have a sad sounding mantra, I remember my LOs mantra changing when our routine went a bit off track, he sounded sad rather than meditative, it never escalated and remained repetitive.
From what you describe, if your LO has not been using a mantra cry, and his cry was escalating it sounds like an 'I need you' cry, to which you respond immediately. Go in, use your key phrase straight away if you don't know what else to say/do. I generally started responding before I even got to DS's room "I'm coming" and "I'm here, Mummy's here" when I got there. LOs are listening out for you all the time.
If he settles down as soon as you are there you can leave again. Mine did not like me to stay in the room when he went to sleep so I would say "I'll go now so you can sleep properly, call if you need me" and leave the room. If her started to cry out for me again I went back. This is a form of WI/WO there is no time limit to how long you stay with LO and it does not mean you cannot combine with other methods of soothing, it just means when he is calm or drowsy you leave so he can nod off.
Going onto his knees is certainly a milestone, developmental leaps often disturb sleep, it's just something you have to go through I'm afraid. The developmental aspect may well be contributing to his crying before naps but lets also look again at his routine.
OK, looks like he is waking later in the morning a few days. This is resulting in his A time being shorter when you put down at the set nap time you can either set his morning WU time at 7.30 to regulate the day or move his set nap time later. With a regulated WU you may find the set naps settle back down or you may find he needs a small shift in the routine as well (impossible to know without trying).
How do you feel about regulating WU time, I get the impression you are more comfortable with the days being predictable and this is one way to make it so. Regulating at 7.30 means either you go in and wake him or if you like you could try setting a lamp on a timer switch to come on at 7.30 so he wakes with the light.
He may still need that extra 15 mins on the A time but honestly it is hard to tell because his morning WU has moved later.
It seems he is still going to sleep well at 7.30pm is that right?
Let me know your thoughts. Meanwhile if he wakes late again before you have a chance to wake him or if you are undecided about regulating WU then go ahead and add that 15 mins onto his A time on both naps to get you through the interim days.