My dd is 6.5 and has been on miralax for about 2 years for chronic constipation. It pulls water into the bowels to help soften the stool. It can take a couple of days to work bt once it starts to work it works great!
The way our dr has prescribed is like this : When DD gets constipated, use it daily until she is going regularly AND it's soft. During this time make sure she is getting plenty of fluids(which helps the medicine work) and increase all fruits and fiber. Once she is regularly soft, then wean her back to every other day, then every two days etc until she is off it completley.
My DD's has had bowel issues since infancy. this has worked the best for her. We hope she grows out of it....

A hemmorioid on a little one would warrent a visit to the ped asap, and until that appt, keep note(if you can) how much and how often your son goes. Also TMI HERE look at it or have your son describe it. This really can help the dr determine the issue.