I'm glad you've managed to carry on like you wanted
My daughter is 9 months and I'm still fully breastfeeding, I also suffered from post natal depression and when she was 5 weeks old I damaged a breast by trying to hand express so now only one gets full of milk and I have to deal with the size issue but I was determind not to let that stop me. I also have a 4 year old son and he was ill when he was born so breastfeeding was difficult and only managed a few weeks so this time I wanted to try as long as possible, but with the depression and how demanding breastfeeding was I did consider giving up. I tried Lily with 5oz of formula a day at 3 months (aptamil 1) but when she started solids at 4 months the difference in feeds she demanded was HUGE! They are so much more content when they are on solids that they demand so much less milk, you can actually have time to do things!! Only thing was as shes got older she refuses formula milk but now I can give her baby juice to substitute while I'm out and about......Basically don't give up and hopefully his weight will improve with the formula/breastmilk combined.
Just curious as you're in Birmingham, where about are you? The sure start centre with the breastfedding group sounds good and I'm in South Birmingham