I just wanted to ask are you sure he's having weight gaining issues? Are you checking his weight on a home scale or at the doctor's office? At the same time in the day, without a diaper? If it's not the same scale all the time it could account for what looks like a lack of weight gain. Is your doctor concerned with his weight? Check all these things first to make sure there is an issue before stressing yourself out. My 3.5 yr old dd always had weight gaining issues from the time she was born. My first doctor totally stressed me out about it and told me to stop bfing and change to formula! Instead I changed doctors
and got a ped that was supportive of bfing. She in fact was not concerned with my dd, she pointed out that both my dh and I are of slim build so it made sense that my dd was in the 10% and less scale. She was not concerned, even the odd time when my dd did not have gains, she pointed out that my dd was alert, happy, and had lots of energy. She also said "well, someone has to be in the bottom of the grid for weight otherwise there would be no grid!".
If you still want to top him up I would not switch to exclusive pumping, it's more work than bfing (from what I've read, I bf my dd for 18 months and never pumped a day in my life). I would maybe just pump the breast that he didn't take and offer that in a bottle the same time (but the day after, so you always have a full bottle from the previous day's pump). I don't think you can actually get them to drink more in a day in total quantity. I think any extra drank at one meal might mean less is taken in at the next. At least, that's how it was with my dd. The only time I was able to somewhat impact her weight gain is when she went on solids as I was able to pick the calories in foods and offer higher calorie foods. While exclusively bf, I couldn't "pick" higher calorie meals.
