Hi there and welcome to BW forums
You may well be seeing SA, it's a tough age for that and you're right that you'll need to give him lots of reassurance and help strengthen the bond of trust between you, so that he learns that he can trust you to respond to him when he needs you, even when he can't see you. Can I just ask how he's currently going to sleep? Do you end up holding him to get him to sleep, or putting him down but staying with him, or leave him until he's asleep, or something in between?
I'm not sure if you followed the link from the article you mentioned to this one:
Toddlers: Walk In/Walk Out vs. The Gradual Withdrawal Method (HOW TO CHOOSE) but it describes both the wi/wo method as applied to toddlers, and also the gradual withdrawal method, which may be better for you if he's become really distraught about being left by you. Have a read if you haven't already and see what you think. If you do decide on wi/wo, please bear in mind that if he's crying really upset "I need you" cries, you don't wait a set amount of time, but you return immediately and do whatever he needs to calm down, ideally not picking up but it may be that you have to. Then back down again if necessary and back out...and back in...etc. Yes it may feel silly but it's really important to teach him that he really can trust you to return and that he doesn't have to worry. Eventually he'll realise that although he'll have to go to sleep on his own, you are still right there and will come when he needs your reassurance - does that make sense? If you try this for a few days but he's still really distraught, you may need to 'cave' and stay with him, help to soothe him to sleep, and then you'd use gradual withdrawal to wean him from that. Keep us posted and we'll walk with you
We don't support any form of CIO or CC (ie waiting set amounts of time before comforting) here but we do encourage listening to our LO's cries and hanging back if they sound like they're settling themselves or responding to them if they need us, including their need for comfort. [As an aside, what we call PU/pd is actually staying with them by their side, giving verbal and/or physical reassurance. We'd only leave them once we know they're settling themselves, and return if they get upset. Neither PU/PD nor wi/wo are based on set time intervals, rather they're about listening and responding to our LO's cries
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On a completely different note, 13mo is usually somewhere in the middle of the 2–1 nap transition. Would you mind posting your current routine - not just what you're aiming for but what actually happens, as we may be able to suggest a few tweaks which could help him to settle and sleep better
Oh, and another thought - did things get worse when you started giving pain meds? It's just that I've heard of some LOs who find that ibuprofen actually gives them really painful stomach cramps; other LOs find it much more effective than paracetamol , so I'm not saying don't give it, just something to be aware of in case it has this effect on him.