Large breed dogs grow very quickly for the first 6 months or so.. after that they seem to "shoot up", but skinny up and not gain much weight.. Its not abnormal that your puppy has only gained 5-6 pounds in the last couple months...
Male dogs will reach sexual maturity around 7-12 months of age. which means he will become a " LEG LIFTER".. My dog is a 9 month old Doberman and he was a "SQUATER" till a couple weeks ago... he is about 29 inches tall, and fully grown he shouldnt be over 30 inches. But over the next year he should start Filling out and and grow his other inch.. LOL
and as to your concern about his " shyness"... Lots of love and attention as well as basic obediece training.. Sitting when meeting new people is an EXCELLENT thing to learn... Also something that helped my puppies... Have the friend stand beside you, not directly in front of you.... facing someone is sometimes a sign of Confrontaion or wanting to fight.... Same as when you are training your puppy to COME... Starring directly into the puppys eyes, is not a good idea.. . I Made funny noises and did silly stuff cause puppies loves to play... Hope this helps ya... Just remember you are in CONTROL and your puppy looks to you for guideance.