Take the larger pups off mom now and then and let the little guy get some extra. The other info you were given is also great!
Move him from teat to teat when he drains one out. Then put the others back in and let them stay there as they normally would. I have taken mom out for an hour or so, let her fill up, put her in with a weaker pup, let it nurse all it would. take her back out, for about 30 mins, then put them all back in with her. You'll get your little routine down pat soon. Supplementing with a bottle is also a good thing to do. But you want him to get as much of mother's milk as possilbe.