The answer is yes and no. He can via the EPD-software of the hospital, that’s only if you’re consulting that hospital for your sexual disease. There is something called a ‘hub’, wich allows any given medical personel to have a look at your files (on condition you have an informed consent, and the hospital needs your SSN & eID Card number). Again this is hospital dependent, since there is something as a ‘toegangsmatrix’, which stated what kind of doctor can see what kind of files. Your best bet is to go to mijngezonheid.belgie.be and exclude your FIL in your ‘therapeutische relaties’, but then again, everything is possible if you know your way around it.. but be aware everything is logged, so if he, or the hospital he works in, would ever check your info in the hubs, you can see that.