Hello Gavin
The main issue with claiming DLA for very young children is being able to prove that his development is behind that of his contemporaries - not always easy, and you would have to have proof from medical professionals to back up his lack of development.
If you go to Members only Guides and click on DLA
Then click on DLA Claims;
here you will find two different claim guides, one for children with mental health issues, and one for physical problems. If you download both of them and read them through carefully you will be able to pick out the descriptors which apply to your child.
I believe your child won't be eligible for the higher rate of mobiity (if he qualifies) until the age of 5; the lower rate might be available from 3 years old.
If you don't find all the help you need in the guides, then members of the forum might be able to give you support and guidance; if you bookmark this page then you'll be able to get back to us easily so that we can help you further if necessary.
Good luck with the application.