Hi Mel,
Firstly I completely empathise with you. My son - nearly 10 months, second child, self soother and night sleeper until 6 months, only a couple of pounds heavier than your son, breastfed, wonderfully happy child during the day.
We didn't want him waking his brother so took him downstairs and rocked him - stupid thing to do we know but it was just easier at the time.
We had tried doing PU/PD, shush pat and taking him into our bed but nothing worked. Turned out we were doing the wrong thing for his age. Over the last three weeks we have been using the altered method of PU/PD - the one recommended for 6-8 months. It took some time and he seemed to get worse before better. He now puts himself to sleep and, for the most part, he sleeps through until 5:30am at least. Basically, this method involves using more of your voice to soothe/reassuring hand on back (but not patting as this can wake them) if speaking doesn't settle/picking up ONLY if the rest isn't working - straight away putting them into cradle hold and laying them back down, saying something like 'It's alright darling. You're just going to sleep.'
I don't know whether this is something worth thinking about - it was just because of the similarities that I thought I'd mention it. Whatever you find that works, I wish you the best!!
Sarah xxx