Some boys need more time, and it will come. My son, now 2 had the same problem. He was crawling at 5 months and walking on his own at 8 months. He started talking more than just mama and dada at about 18 months. He still has a pronounciation problem, but it is getting better.
It is more they are wanting to move rather than have an interest in talking. We spend more time reading to him, and the more learning type shows - Disney, Nick Jr., PBS etc. he watches the more words he picks up. We have noticed that he has the vocabulary, chooses not to use it, and will even surprise us with long hard words or phrases such as awesome, serious, fire in the hole (hubby tought him that for when he poops his diaper), and he comprehends so much more than we can imagine.
Your son is just choosing not to use his words. encourage him to use his words when he wants food, or something to drink rather than just giving it to him. Or try having him repeat what you want him to say, such as "tell mommy, 'I want milk". Say one word at a time and wait for him to say the word, you may have to repeat the word several times. Also spending more time talking to him will help. Don't let your other children speak for him or just give him what he wants, that will only delay him speaking. Make them talk to him, ask him questions and then have them help him with the answers. It will help, but really don't push him, it will come, and when it does then work with him more.
good luck, and know you are not alone! It is frustrating, but give it time and it will come.