My ds #2 is over 20lb, almost 28inches long & only 4.5 mo, my ds#1 was the same only not as long...so I am with you on the big size.
If you personally don't feel ready... read this, if you want to go for solids don't let me put you off.
What I wanted to add was just because your Dr says your lo should be on solids doesn't mean he has to be (Drs aren't always the best people on "healthy babies")... if he is doing fine on breastfeeding then I would say don't stress yourself about solids. There is ABSOLUTELY no benefit in solids before 6mo & World Health Organisation actually recommend hold off as long as possible due to the risk of increased allergy & intestinal disorders.
Also in younger babies solids can actually increase night wakings, NOT decrease if their intestines are not ready, if that is a problem with your lo.
If you feel your lo is not getting enough feeds from daytime bf it may well be that he is distracted & not taking full feeds. I always give Liam 5mins & re-offer my breast after he comes off a feed & about 50% of the time he takes a good feed 2nd time around- he sleeps about 10hours between feeds o/n
I don't plan offering solids to Liam until he is 6mo (maybe longer) & I lasted until almost that time with Aiden & he weighed 10kg (22lb at 5.5months) & slept 8hours a night, so it is very possible for very big babies to be totally bf & happy