My baby isn't sleeping well

Sara
My little one is going to be 5 months on Monday, he used to be such a good sleeper
Sleeping all the way thru until the morning but lately it's all changed!! We are having a nightmare getting him off to sleep and around 3am he's wide awake or screaming in his cot!! I tried the whole putting his dummy in and sticking the mobile on and it works for an hour and then I find myself up again to him, I end up putting him in bed with me and my partner and he's fine!! I don't want to keep doing this as he will get used to it... Can anyone suggest anything ?? Some people say it could be him teething which we think is a possibility !! Any suggestions I would be grateful ... Thanks