Hi Mower
My daughter is 13 weeks 1 day. From week 8 till week 12 we had a similar situation. From week 7 onwards she slept slightly better, up from waking every two hours, to three. Then to twice a night.
From week 8 she slept five hours a night but basically yelled from 8pm till midnight unless I constantly nursed her.
There's several possibilities and excuse me if I'm teaching my granny to suck eggs (I'm assuming you've checked for wind, nappies, hunger etc):
(1) He's overtired and needs to sleep
(2) He's got reflux and whenever you lie him down he's in pain
(3) He doesn't know it's time to go to sleep.
You don't say how much nighttime sleep you are getting from him. If it's that he sleeps from 12 - 5 and that's your lot, that may well unfortunately the longest he can go at the minute without food. Sorry . But that doesn't mean he can't nap before his longest sleep.
If he isn't having several long naps in the day, that may mean he's overtired.
If he is fine if he is over your shoulder but as soon as you lay him down - even if you are lying down next to him - he may have reflux. See your GP. Elevate the head end of the cot.
If he is fine to lay down and feed and then can lay on the floor quite happily, it's probably not reflux.
If you don't have a sleep time routine. Get one, quick. It helps your little one wind down and get ready for sleep and trains him when to rest.
Ours is as follows: 5pm milk time, 5:45pm bath, 6:15 quiet storytime, 6:40 swaddled and made comfy - dummy given if she wants it. 6:45 cuddles upstairs in a darkened room, only shushing, not talking. 7:00 into cot, pat to sleep. If she wakes, we pick her up and pat her and shush her but NEVER take her out of the room. If she's still hungry and won't settle, I feed her again and then go back to cuddles and shushing and into the bed. At 10pm I do a dreamfeed (ie feed her in her sleep).
Naptime we do pats and shushes and swaddling and she does go down. We're only starting off and we've only just had three nights of properly sleeping through (ie till past 5am -she now sleeps 10-7am then feeds and sleeps till 9am (and then feeds - we reckon she's sponsored by the milk marketing board).
But it does work if you stick to it.
Can I suggest a couple of books? Tracey Hoggs, the Baby Whisperer and also the no cry sleep guide. Really gentle and effective together they've saved me from the living hell that is a sleepless infant.
Oh and btw, I found that weight was also a tipping point, once she hit 10lbs she slept much better because her little tummy was so much bigger.
On the problem of him wanting to be carried, again this is an issue of working out other strategies for getting him sleepy. I had my daughter like this for a while and what I tried (and it worked for me) was to instead of putting her over my shoulder, putting her down in a bouncing cradle so she was reclined and sitting on the floor next to her and alternately (ie mixing the two strategies), turning her so she was being rocked in a nursing position (NB don't do this if your son has reflux, he'll be sick on you and be in pain). Having two different ways of doing things meant she didn't get used to (and cured her reliance on) the over the shoulder method.
Good luck
Rachel