What Causes To Pass Bowel Movement Only Twice In A Week?

I am almost at the end of my rope with my son's bowel habits. He is 7 years old, and in typically only has a bowel movement once every 3-4 days. In the days in between, he will go to the toilet and push, and usually create a huge mess in his bottom. The poo will be very thick and messy, many times creating a mess on his legs, sometimes underwear and toilet. I usually clean him up because I fear that if he tries it will only create a larger mess. I feel like at age 7 he needs to be able to have healthy daily or at least every other day bowel movements and he should be able to clean himself up. I've tried Miralax, prune juice, decreasing milk, and increasing fruits and juice. Nothing seems to work. Any suggestions?