Hi. Ive had a bunch of experience with stuff like this in my family, so here's my more than two cents. Hope some of it is helpful.
1) never give a laxative to a child without consluting your pediatrician!
2) yes, water is always helpful, never harmful! If he doesnt drink water at all, start there. If he does, up it some.
3) He might need more fiber, which comes naturally in fruits, vegetables and whole grains. If he is a steadfast stick to my favorite foods kind of kid, incorporate these into your son's diet slowly. You can look on line for creative ways to do so.
4)It goes against my intuition, and what many pediatricians and older parents have told me, to mess with the workings of a child's digestive system unless they are in pain or obviously ill. I would put the question to you as the one who knows best, but might not have total control, over what your son eats - what gives a human being their level of gas and its smell or frequency depends almost completely on WHAT THEY EAT, with a bit of the factor being if they are battling something like a cold (since all systems are connected, digestion can go out of whack if the immune or sinus system is depleted, there can be a lot more mucus, etc).
That said, I think it is up to you first to look at if there has been a change in his overall health, diet, or both since the problem began. Try to ask him about this as delicatedly as you can. (New treat at a freind's house? etc). If it is not that, I do realize that a lazative is the easy way out, it is quick, involved minimal coersion on your part. However, this is invasive. Your son's gut is not your friend's kid's, it is his own, and what worked for her kid may harm him.
If none of the above work, and you want to be gentle to his gut and have a bit of energy, try an elimination diet: taking out one problem food at a time, then reintroducing it after a few days, observing any changes that you see. (With adults, it is best to take out ALL food that could be the problem, then reintroduce them one by one, but that's never easy with kids and could lead to great resistance). If you are going to do this, bring him in to the process, so he doesnt feel like your taking away sugar for two days is punnishment for something: If he is into gross stuff, science or both, you can bring him into it with an experiment: today and tomorrow no dairy, and you can write down how many times you fart in this notebook, we'll call it your fart journal." The foods that are big culprits of gut stuff are:
1) dairy
2) sugar (sorry)
3) meat
4) fried food
5) wheat products (the culprit here is gluten, the protien in wheat)
Good Luck!
Alex