This is really traumatic for you, and to be in a foreign country at that...
I would continue to pursue answers to this unpleasant riddle. It's not too uncommon for toddlers to have seizures with high fever, the very issue that we are warned about in keeping the fever down with cool baths, towel baths, ice chips in the mouth, some folks rub alcohol on the skin (I never did) etc...
It sounds to me like your child has a virus, what kind would be difficult for many doctors to say. It's too bad he was misdiagnosed with Pneumonia, it could have been the mucus rattling around from the possible allergy reaction... Parasites are not out of the question either, which can be easily picked up at beaches, playgrounds, on hikes, or even from food or beverages. Don't rule these out.
Try to retrace the things you did and the places you went the two to three weeks before your child got sick. Viruses and parasites take time to get to your child's health, and sometimes answers can be found by retracing your steps. Insodoing, are any other folks or children you came in contact with during these past few weeks sick in any way (their symptoms may be different from your son's)?
Nevertheless, I would not drop this even when the boy's fever goes away. You must have a US consulate nearby, or get in contact with some US expatriates like yourself to see which doctors they go to. (I am a foreigner to this country, been here for years, but there's nothing like going to a physician whose practice is most similar to what we are used to. So it's not that I'm saying Brazilian doctors are incompetent, no way) Find a US trained doctor who speaks the same "medical language" as you do. I hope you understand what I mean....
In the meantime, if your child persists with the fever, keep him cool, keep his body temperature down. I used to get beach towels wet (not drippy) and wrap my children in them and change the towel out when it became room temperature, keep a bowl of ice water by the bed and put a soaked washcloth on their heads or necks, wherever it made them feel better. Some of my friends would stand in the cool shower, or get in a cool bath with their kids...Also, two of my kids don't respond to Tylenol (acetaminophen) but respond wonderfully to Motrin (Ibuprophen), (don't give him any aspirin though! way too young for that) so you may need to see what your options are there. Just make every effort to keep his temperature under 101F. Home made popsicles were good (just make sure to keep sugar intake down when he's sick, even fruits can be too heavy when they're really sick like this) chicken soup and rice, keep the meals light, drink as much water as possible, look into giving him electrolytes, ...make sure he's properly hydrated.
I hope someone gives you the advice you need R.. My heart goes out to you.
In Friendship and Support,