What proof would you need to support your cause on a soldier malingering ?

You have to tread lightly here. I once had a NCOIC try to press me on malingering. The backstory on that was my wisdom teeth. I had heard horror stories about getting them all out at once so I started having them removed one at a time. I was in DC at the time and with join bases the AF dental facility was an easier movement, I later learned they gave quarters for even small things. So over a period of 2 months I had surgeries and 4 sets of 72 hour restrictions to quarters.

As with all other things medical, any concerns need to be addressed with the coc and the medical professional. Only the professional knows if they SM is legit or not and if in doubt, there are other tests they can run. With stomach issues it can be anything from nerves, GERDS, or more serious issues.

My cousin was caught malingering by his PSG and platoon leader.They started making barracks checks of all soldiers who were on quarters or as my cousins case were issued crutches for leg problems.He was caught out at a beer joint dancing when he claimed he could not walk well enough to do his job.Finally got a general discharge under less than honorable conditions.Good riddance.I thought.

Your command needs to speak with the clinic, from there they need to discuss his issues within Hilo’s standards. Example, that many stomach issues warrants many different evaluation, and assessments to determine the root cause. All the while you keep notes and copies of quarters and profiles. After exhausting all venues you put together the paperwork presenting it to your command and JAG. The rest is up to Commands willingness and JAGs guidance. Thank you for your service.

I am going to recommend that you treat this as a genuine rather than a put-on medical situation for two reasons: first, it allows you much more leeway in investigating the situation, and second, if your suspicion is wrong and they really are sick, you still have their trust and goodwill as you try to get them sorted out.
If this soldier is yours, have a good look at the sick call sheets to see if it is the same specific complaint every time or something kind of different each time. The first would make me want to ask, "why isn't the Doctor following up to get to the bottom of this?" The second would make me want to ask, "why are the staff at the TMC allowing this to go on. They're usually pretty sharp at spotting fakers."
If the soldier keeps getting quarters, perhaps you should ask your 1SG to officially enquire of the TMC the health status of this soldier. This is something Top is responsible for and will probably be all you need to do.
If the soldier really is sick, he'll be off to the hospital for evaluation. If not, Top will have his hide. It will just sort of "happen."

Isn't a written ledger still kept officially somewhere when someone goes on sick call so their time can be accounted for? I thought at least BN Sick call maintained such a ledger. If so, just get a copy of the ledger and start documenting. I think you only need to tell one person in the BN Aid station to use that approach.

I would try to rule out that he doesn't actually have a medical issue. It sounds like Lactose intolerance. Symptoms would include stomach cramps and runny poo. He can keep a diary of what he eats and when it happens; that would help a doctor assess if he has an intolerance or allergy to something. If it's lactose, he can likely just pick up Lactaid/Lactaiz over the counter and then he can keep eating dairy. I just went through something like this with my older son.

Tread carefully

I got a troop for Mali getting once. He was on a no pt profile. Cool, no problem if he was hurt. I went to the gym and saw him playing basketball. Took some pics and went to his doctor. The soldiers profile got voided for if he could play sports he could pt

I had this same issue with a soldier very recently. Come to find out she was diagnosed with Crohn's Disease. Tread very, very lightly. Get copies of sick call slips and have your PSG/NCOIC get with the company commander to contact the SMs PA.

You are more likely to get yourself in trouble than solve what you perceive to be a problem.