Foremilk/hindmilk

I just took my 5 month old to a new doctor today and he asked how long he nurses for because he is not gaining enough. He was born 8.6 and he is 14.5. He nurses for about 5 mins only per breast and doctor suggested to feed 15-20 mins each so he can get the fatty milk. My problem is he doesnt want to stay for that long. I’ve been wondering if im not producing enough

I switch sides because he doesn’t want to latch anymore

Any tips or advice to get him to eat more/gain weight?