dont be worried. every child is unique and hence his devt in different aspects will also be at different rates. your baby is still very young and has lots of time for further growth and devt. what your friend's baby is good at doing, your son may not be good at now, but it's only a matter of time. besides, your son may be good at other things, but you just need time to observe and also help him to express those skills. i think as parents, one of our responsibilities is to observe patiently, encourage, nurture and facilitate those skills in our babies, but let them take their time.
just to share: my boy just turned 10 months old two days ago. he can understand many words that we teach him by repeating and pointing to the associated objects, but he has not picked up any baby signs consistently even tho we have shown him the DVDs and keep doing some signs to him every day. he also has not started speaking any whole word distinctly although he babbles a lot every day and we can hear his aural vocab getting richer day by day. he can scoot around and do commando-crawl finally after a long time of self-practice, but he hasn't figured out how to crawl. he loves to stand up but hasn't figured out how to shuffle his feet to mimick taking the first steps. for a couple of months, hubby and i have been trying to show him how to nest and stack his toy cups together and how to play the shape sorter and stacking rings. but he's more interested to topple the stacking cups and rings or mouth the cups and rings, haha!
so we figure that our boy has certain strengths and weaknesses, and we just have to be patient for the areas where he's not so strong or fast in. never once do we think that our baby is not learning or not interested in learning or is not capable of learning. in fact, babies love to learn as everything around them, even the simplest of things, is fresh, new and wondrous to them! but different babies, as adults, have different ways of learning. playtime is learning time too!
i can understand your anxiety, but i would suggest to stay cool and continue exposing him to different things daily, observe him and help him along. i am sure he will surprise you soon and your efforts will pay off. =)