First ! genetics....pls chk the sire and dam of your lab...how are their physical appearance [ tall or short]...then you know what to can expect from your lab as it grows. I have seen many labs ....all are different. Its all depends on the bloodline and breeding. You would probably seeing many labs around and comparing it right !!
As Sunil has rightly pointed exercise is must with a very good balanced diet. One friend of mine has advised me that you need good running exercise for labs or swimming if possible. This will make him more muscular and toned and taller. Include one source of protein in your labs diet daily strictly....Protinex powder [recommended by my vet] can be given to labs...its also good for them as it does not have sugar...mix it in milk + add water in equal qty]
My vet advises brisk walking in morning and evening is also very good to keep labs in a lean condition.
Playing in the park with will keep them well exercised. Labs will be full grown in 1.5 to 2 years. You have a long way to go yet. Give good vitamin supplementation. Do not overfeed your lab.
Keep your lab in a lean condition