I need some ideas on how to get DS to start speaking using 2 words please

[TinySocks]

DS is 3.2yrs and has developmental delay due to brain injury.
He has been saying single words for about 8 months now. He has a very nice long list of words and understands plenty, but he refuses to put two words together.
For example:
DS says: Come
I say: Where
DS says: Up, kitchen, garden, etc

DS says: Want
I wait and wait, eventually I say: Want what?
DS says: Food, water, etc.

DS says: Watch
I wait and wait, and then I say, watch what?
Ds says: DVD

However, we often sing a HELLO song, and he can say "Hello Daddy" when we sing this song. I have tried using the same melody for other two word commands, but it is not working.

I am constantly saying two word phrases to him, everytime he picks something, or looks at something I speak to him using simple language: yellow car, big ball, soft bed, hard floor, want food, nice food, cold water, etc

What am I supposed to do to get him to speak in two words? Any ideas? I am supposed to wait until it just happens or can I help him a bit?
Thank you for any ideas!

Hi Tclanger, thank you so much for the info. I have checked the helpwithtalking webpage, I am a little worried about looking for therapits or professionals without personal recommendations, specially as we don't live in the UK, but I will try to find out more. I will check your blog during the next few days to find out more about the two day assessment. I would feel so much more confident if I had someone guiding us a little more.

Tiny - grill them about how often they deal with kids with your DS's type of problems - whether it's on a daily basis, weekly basis whatever, and how much they deal with speech delay rather than pronunciation problems. also ask about whether they often use AAC (alternative augmentative communication) with kids they see.