how doing get back to being able to put him down sleepy in his cot?
In terms of sleep physiology, it isn't a case of "get back to", it is more than you now need to teach your baby and before you didn't.
In the newborn fourth trimester phase (up to about 3 or 4 months old), sleep is a passive state. As long as all needs are met, then baby will sleep (just as was the case in the womb). Being awake happens when a need is not met (usually hunger) and then once that need is met, there may be a short period of active time awake but baby will soon return to their passive state of being asleep.
Then babies come out of the newborn phase and from about 3 or 4 months old sleep physiology changes, it matures. Sleep becomes active rather than passive and it matures to be like an adults with sleep cycles including periods of light and deep sleep with small breaks between sleep cycles.
It is at this age that the process of 'getting to sleep' becomes an active process - something you and baby have to actively work at doing. In comparison to the newborn phase where little is needed to get to sleep because it is passive, this feels like a regression. It isn't a regression, it is that the process of sleep has changed permanently and now needs to be actively taught.
It is at this age you need to start thinking about sleep triggers and mechanisms and trying to teach baby to sleep in a way that is sustainable long-term.
Realistic expectations in terms of sleep development to come may also be helpful. Babies start to learn to link sleep cycles from around 5-7 months, so don't expect more than 30-45 minutes sleep for daytime naps. Teaching baby to re-settle back to sleep when waking is good practice, but don't expect it to be successful very often yet.
Also be aware that children don't have the emotional development to go to sleep like an adult (tired, lie down, close eyes, relax, sleep) until school age. Until then baby needs some additional factors to aid sleep - most significantly to feel comforted and secure.
Now is a good time to start thinking about introducing that comfort and security that will be needed long term. Some means for this comforting a child can learn to do for themselves - suck a dummy, snuggle a teddy for example. Other means of comfort need an adult long-term (feeding to sleep, cuddling, rocking in arms) and you need to consider if these are things you want to be doing through until your child is 4 or so years old.
Again set your expectations - you will need to teach baby to go to sleep and you will need to help baby gain comfort until they are at an age where they can access that comfort for themselves independently - that might be 7 months old plus. Have the long-term view in mind.
Do you have a nap routine and how long is his awake time?
Fifi speaks wise words. Watch and limit awake time, rather than time asleep. 4 months old wants about 60-80m awake time between naps. Expect naps of 30-45m.
This will rise to 90-120m awake time between naps once naps start regularly extending past 1h.
What are his naps like at the moment?