
You can find the breastfed baby growth charts
here. Like pp's have said, overfeeding an exclusively breastfed babe is nearly impossible... you really can't "force" a babe to nurse if they don't want to, and given the amazing feedback loop between mama and babe, comfort nursing isn't a (nutritional) problem either. I'd say you and your babe are a glowing example of the beauty of breastfeeding to meet a child's very unique needs... can you imagine the stress you might feel if you were giving this munchkin artificial baby milk? Then you really would need to figure out if he's getting too much or not enough!
My dd1 was 9lbs6oz at birth and she has stayed happy and healthy and active in the 95-99th percentile since then. My dd2 was 9lbs at birth and dropped like a stone through the growth chart till she found her own happy, healthy, and active curve close to the 10th percentile for weight and the 50th percentile for height. And my four month old ds? He started at 9lbs8oz at birth and is a happy and rolly-polly 20+lbs at four months.

Given that 2.5yo dd2 is only 28lbs, the two of them together puts a smile on my face and is a great demonstration of "every child is unique".
I find the ergo is a godsend for my back. DD2 required more or less constant carrying for the first year due to sensory processing problems and the ergo was the only carrier that worked long-term. Perhaps you can find a gently used ergo if the cost is an issue? Our ergo is now on child number three so they really do hold up! With a bigger babe you wont need any of the infant extras, though the new "teething covers" may be worth it.
I also have a "soothing motions glider" from fisher-price. It is a semi-reclined bassinet (so ds can see the room, not just the ceiling) on a glider track. The bassinet can spin so it glides side/side or front/back. The sides of the bassinet curve up a bit so it feels more secure and ds really enjoys being in it for 10-15 minutes at a time (especially if he can see out the window as well as see me or his sisters). I do hold him a lot, but given two other young kiddos to care for as well as his weight it's nice to have a bit of a break.
For the holiday we were given a seat (similar to a bumbo) that we may use every now and then once he is a bit bigger... it converts to a booster seat so it's something that will be useful for a while. I know there is a lot of controversy over bumbo, and it's certainly not an every day type tool, but perhaps something like that may give your arms a break too.
Good luck and strong back vibes to you!