My son's language development is very slow as well. My family has been speaking with him in English since he was born. He only watched English TV, English music and books. He attended English playgroup. However, when we (i mean, family, friends) talked with each other, we used Cantonese. (now, I realized that even though we never talk to him in Cantonese, he pick up our conversations and words by hearing it; he understands, everything is inside his brain, but we didn't realize until recently).
He started to pronounce some simple words such as 'car, cow, boy' right after his 2 yrs old birthday.
When we asked his name during 27 months, he would say 'mark' (his name is Marcus.
When he started nursery school, it's getting better. In his nursery last year, it's 1 1/2 hr English, 1 1/2 hr Mandarin. I never speak Mandarin with him at home.
I can say that his language development started to mature after his 3 yrs old birthday. He started to speak up and it's very very funny now.
Since he is old enough to understand different languages now (3 yrs 7 months), he started to learn Japanese & French few months ago. Really surprise, he pick up these two languages faster than me.
During X'mas holiday, I spent lots of time with him at home. I explained to him that Mommy also knew Mandarin. I tested and found that he knew lots of words and understood my Mandarin conversation.