(((hugs))) good for you for sticking to your commitment to BF - bugs me so much when Drs are pushing formula. Although, at this age if you were desperate to give him more I would prefer formula top ups to early weaning, I think? It's a personal preference I guess.
I'd suggest taking some steps to optimise your milk supply, so as to make sure he is getting as much milk as possible at every feed (although you're right the short feeds are almost certainly that he is getting more efficient). Things I can immediately think of are a Fenugreek supplement, a bowl of oatmeal every day, pumping after feeding to stimulate supply. Also feeding him immediately when he wakes from his naps, in a dark quiet room, so that he is still drowsy and undistracted and likely to feed for as long as possible.
You could do this for 2-3 weeks and if you're still unhappy with his weight gain, review your decision then? He's still gaining weight - they do go through patches where they grow fast, then it slows down, then speeds up again - the important thing is to look at the rate of his growth over a longer period, look at months together rather than individual weeks.
what do you think?