He naps every two hours for 30 min in the day
Do you mean 2h awake, 30m sleep, 2h awake, 30m sleep... etc?
Or do you mean 90m awake, 30m sleep, 90m awake, 30m sleep... etc?
At 4 months old I would expect 60-80 minutes awake time between naps (which may well be short, 30-45m is normal at this age) to avoid over-tiredness. Generally not having enough sleep over the course of 24 hours could be a cause of unsettled nights. So you could try getting baby to sleep sooner in the daytime, less awake time between one nap and the next.
He goes down brilliantly at 6.30
What is currently happening at bedtime?
What happens to get baby to sleep a naptime?
Please help or at least tell me I am not alone and it will get better?!
Cold - 5w ongoing is too long for this to be down to a cold.
Teething - You cant medicate open-endedly. The teeth actually cutting through causes the significant distress.
Hunger - he could be. Try feeding very frequently through the daytime to calorie load. He could like sucking - try a dummy?
Weaning - Far too soon.
Settling - Always a good thing to get into the habit of trying to resettle
Phase - Life is a series of phases. We have to change to accommodate these changing needs.
You are not alone that sleep changes completely at 3 to 4 months old. This is a physiological change. Baby's calorific need significantly increase. In my experience I was feeding 25% more than the newborn phase.
Also sleep changes significantly. It stops being passive and becomes active. In the 3 months baby's sleep is like in the womb, a passive state. If all needs are met baby will sleep and baby will wake when a need (food, nappy etc) is not met. Sleep requires no thought or work, it is the default.
From 3-4 months old baby's sleep matures. It develops into cycles (similar to an adults sleep) with periods of deep sleep, light sleep and sometimes 'environment check' brief wakes between sleep cycles. Sleep is no lonher passive - it requires active work for baby to get to sleep and for baby to stay asleep during the lighter sleeping phases in the cycle.
Previously, the concept of "getting to sleep" was unknown to baby. Now starts the hard work. Developing ways of helping baby sleep can become all-consuming for a while.