Have you had your child checked to see if they have a mild tongue tie? My second daughter was born with a mild tongue tie and although it didn't affect her feeding she has trouble pronouncing 'k', 'd' and 't' sounds. She is 4 1/2 so not that much older than your son. We have been aware of the issue since she was younger and have been helping her to try and say the sounds she has trouble with.
We have been referred to a speech pathologist but in the meantime have been using the activities I found on this web site.
http://www.speechtx.com/articulation.htm
We have seen a huge improvement in her pronunciation and she loves doing her games each day. I found the site by googling "speech pathology pronunciation" and there were a range of sites to choose from.
Hope that helps


