My 4 month old crys when i put him to bed

my son is 4 months old and crys everytime i put him down to sleep. I had the same problem when he was first born until he was around 8 weeks. Then after that he would go down for around 3 hours or so before waking. 

But the last couple of weeks everytime i put him down to sleep we crys. I have tried putting him down once he is asleep and he just wakes and crys. And i have tried putting him down awake but neither seem to work. He will not take a dummy. As soon as i pick him up and give him a cuddle he goes straight back off to sleep. 

The only way i can get any sleep at the moment is if he is asleep on me. 

Any suggestions please.