Babymum is he on a sleep programm...or do you just put him to sleep when you feel/think he is tired?...my daughter is slightly younger than your LO but she would hardly sleep over the da . I just through she is super super active and does not need that much sleep....but at times she would still be crancky....and cry (even though in my mind I ticked all 'to check' boxes like feeding, changing, etc.). Even though she would hardly get any sleep during the day (1 hour...maybe 1.5 hours cumulated from small naps) she would often wake up at night and would not sleep through the night (6-7-8 hours in a row).
I then stumbled upon a great book by Polly Moore (The 90-minute baby sleep program) and realised that she was underslept and the reason she couldn't actually sleep was because she was so so tired. I have decided to give it a try and she's been on this program for aprox. 2 weeks and the results are just amazing. Now she takes 3-4 naps (of at least 30-45 minutes each) daily and at night she goes to bed around 20.30-21.00 and she does not wake up (unless she needs a diaper change) until 6 - 6.30 in the morning...... I find her so so much happier....and you can imagine how happy I am
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I am also in the process of teaching her to fall asleep on her own (and I use as a guide the same book) and she seems to do just fine. She hardly cries/moans for 5 minutes (or less) before actually falling asleep on her own in her cot....I believe this is because she started associating her tiredness with the actual need of sleep.
The book focuses more on the first year of life of a baby...but does provide some very helpful tips for toddlers (for their parents I mean) and the way they should recognize particular patterns when it comes to sleep and handle them appropriately.
I hope this helps....