It sounds like your son's hip flexors are unusually tight. A chiropractic consult and adjustment could prove very helpful! Dr. Chris Creed is extrodinary and has helped me! His number is ###-###-####. He also does housecalls which might be helpful with such a young one!
He works with people a long time until they are comfortable so they are able to be adjusted properly which is important! I went to him for problems with my feet and I am extremely ticklish! Really, like a two year old - it is pathetic! For this reason, I have a hard time having people with on my feet. He worked with me for almost an hour so he would be able to adjust me - I'm walking fine now!