I wouldnt be atall concerned about him talking,altough he does seem a very late walker,all three of mine were walking at ten and half months,so to me 18 months does seem very late.It just seems he has found another way to get about,and it suits him for now, im sure its nothing to worry about ,you could always check with his health visitor to put your mind at rest.

What does his pediatrician say? My oldest didn't walk until 15 months, but I think 18 months is the outer limit of "normal" and he should have a complete developmental assessment, especially since he's "walking" on his knees.

Not talking can be less worrisome at 18 months.

Good luck.

It's a bit odd. Could mean something. But also not...
In what other ways is he out of the ordinary?
Ahead? Behind? Personality quirks?
Does he have frequent ear infections?
How are his other physical milestones?
Normal pregnancy and delivery?

Just to be on the safe side, help him learn to walk by holding his hands and supporting him gently. Play jumpy-bouncy games to build leg strength and observe his balance. Help him coast along the furniture by providing safe and soft areas to fall on.

Also try to encourage meaningful verbal responses. "Do you want milk or juice?" then wait for a "M" or "J" (may be a "D") sound. Show him how to make some sounds by pointing to your mouth and making the sound slowly and carefully. See how many sounds he can make, and praise him whenever he uses them with intent. (Other than screeches or 'all-purpose' grunts.) Play games with him that encourage him to verbalize repetitive sounds -- like cars and say "Go, go, go" - encourage him to do the same. Again, praise all of his efforts and pull the most he is capable of. (In other words, if he is able to say "Mi" for milk, you 'don't understand him' if he just grunts and points. Ask him to say "Mi". If he can't say "Mi", then don't deny him the milk.)

What does the Dr. say? Have a specialist check his hearing NOW"!!!!! It could be water behind the ears, and large tonsils and adnoids. My GD had this problem twice at 2 and not my Great GS is having surgery tomarrow for this problem, He walks ok (runs) but refuses to talk and we know now that he does not hear good. A very good PA + great grandparents insisted . PA said it is like he has his hear under water all the time. Very common, but some pederiations refuse to acknowledge this. He also could be slightly austic, but a specialtist is needed.