So, I don't think, actually, you need a special 'parasite doctor'. This requires a stool test. A GI doc would work fine but even a family practitioner could do this. My family had a parasite issue, I'm sad to say, several years ago. We visited a farm for a family reunion and although no one drank the water as it was from a cistern, someone washed fruit and made a fruit salad that we all ate. Over 70 people got parasites from that trip. I knew because I had horrible gas, a lot of diarrhea and lost weight. My husband was diagnosed first as he had the same symptoms. Then my at the time 1.5 year old toddler and then me. We had to bring a stool sample into the office. We collected it at home. Ask for instructions for how they want you to do it. Sometimes they give you a kit that you pick up and use at home and then return and sometimes you just collect it. Not a glamorous moment in your life, but it gets the job done! We had giardia. We had giardia. https://www.cdc.gov/parasites/giardia/index.html Very common. You take antibiotics to get rid of it. HOWEVER, and this is important to know, you can get better on your own. My son, the toddler could not keep the medicine down. And I didn't take any as I actually had a newborn and was nursing. Mild parasite infections can get better on their own within a few weeks. I was indeed better a few weeks after diagnosis but my son was a little longer as he got a second parasite! Darn the luck. we worked with the health department which tracks these events. They traced (they are good!) to our having gone to a park and fed ducks. Apparently that is a big way to get parasites. You pick up rocks to throw into lake near ducks, you can get feces on your hands. That goes into your mouth, wa la, parasite. The tracing from what we did after that led to one situation we could have spread it and it was swimming in a community pool. A blown out diaper in a pool can do the same thing. You have to hope that they keep the chemicals high enough in that environment to kill anything like that.
Anyway, what symptoms is your boyfriend having? I'll chat more if you share more information.