Hi,

It can be very worrying when you get that sort of advice from your doctor - especially if your were thinking things were going well (and basically they are)
In the early days many babies are content with one breast. But at some point they probably need both. As others have said, after 20 mins or so (there is no set time) take him off one breast, give him a burp and a nappy change and offer him the other breast at ever feed. You will be astonished at how much of a difference this makes to his weightgain and your milk supply. pretty soon he will be demanding both sides
Interestingly, your baby sounds pretty normal to me. Unfortunately the advice your doctor has given is advice that will lead to premature weaning of your baby.
Early solids are usually not the sort of food that puts meat on your bones - more like what you would eat if you were on a weightloss diet - not a weightgain diet - stewed fruit and rice cereal. He needs more breastmilk - and when he is older he can have normal family foods