HI THERE HOW ARE U?  WE ARE GOOD?  WELL WE ARE LACTOSE,TOO.  BUT DIARHEA DOES FOLLOW WITH THAT AND VERY BAD PAIN IN THE STOMACHE.  HOW OLD IS YOUR SON?  CAN U ASK HIM IF HIS TUMMY HURTS.  SEE I CAN EAT ANY DAIRY PRODUCTS BUT NOT MILK CAUSE THERE IS A SUGAR IN MILK THAT SOME PEOPLE ARE ALLERGIC TO.  SO U JUST MIGHT WANT TO WRITE DOWN HIS DIET AND SEE WHAT IS MAKING HIS TUMMY UPSET.  NOW MY SON HAD TO BE ON PROSOBEE(FORMULA) BUT NOW HE CAN DRINK REGULAR MILK.  THEY SAID THAT HE MIGHT GROW OUT OF IT AND HE DID.  SO TAKE HIM OFF HOW DAIRY COMPLETELY AND SLOWLY FEED HIM CERTAIN DAIRY PRODUCTS AND SEE WHAT HE CAN'T HANDLE.  THERE ARE DIFFERNT TYPES OF LACTOSE,GOOD LUCK SWEETIE. 

Are you noticing this only lately, or has it been throughout his lifetime? If it's just recent I would first assess what changes may have been made recently in his diet, or if he is teething - depending on his age, if he's been exposed to a little bug, or is in some way mildly sick, etc. If it is not just a recent thing then I would take a couple steps.

1) Monitor his diet and bowel movements. For a few days to a week write down what he eats and when he eats it, you don't have to measure it exactly, but if he has a handful of cheese cubes write down, "handful of cheese cubes", a cup of milk write that and what size the cup was (small, med. large) etc. And then when he eliminates, and what consistancy the fecal matter was. Loose, watery, solid, etc. It's important to write days and times with both of these things.  

2) Make a doctors appt for him after you've monitored his food intake and bowel movements. Take those notes with you. They will help your doctor see any correlation that there may be between diet and his diahheria. And from there you in conjunction with his doctor can decide what to do, whether it be medical tests, or experimenting diet wise with him, or maybe waiting it out.

It is possible he's inherited your lactose intolerance, or his body may be reacting to something entirely different.

I'm assuming u mean lactose intolerant?

Dairy products of any kind will go through me. To find out if u are lactose intolerant is to stay off all dairy products for a few days, then introduce one, usually milk, back into your diet. If it goes through then it is a pretty good indication. A visit to the doctor is a good idea. These sort of things can be passed down to our children.

I would talk to your family doctor to see what he/she says and what are other signs to watch out for. I had to do that because I am also lactose.. anything diary related goes right through me.