if you are comfortable with the idea of the safety issues being covered.
ie fil wont give suspect food, and would be a trustworthy person to have the care of your child, you need to give this some thought.
If you are going on holiday again, you could make sure your child has something,a big something , out of the ordinary for you to give him, when ever unkind family rub this allergy stuff in.
for instance at easter my son often ended up with a huge egg , because the smaller treats were out of bounds.
there is no harm in anouncing something along the lines of praising your child when he doenst react to his fil's poor choice of phrase ( the bullying) and hand him something. praise in such a way that fil is shown to be a fault, for instance,
how about saying,
'i know you (yr son) cant that that ice cream, you understand your allergies, and that you cant have egg, well done for not making a fuss. you know that you can have ----? later
theres nothing wrong with you joking with fil, for instance when he offered to swop the sandwich to your child, you could have said, 'here , swop with me, I put rat poison in this one, its up to you to drink some water with it'
and then you could turn around to your son and praise him for saying no to a grown up, because thats a very difficult thing to do, and being aware of not eating any food that mummy and daddy have checked.
Keep this in the same jokey tone that he has, and then with luck he might pause for thought every time.
on a reality check though, as your child gets older , this will be the norm for your child. There will always be people who eat food he cant around him.
I can understand you wanting to keep this sort of thing low for his early years.
once they go to school, and senior school, and adult life, this will be the norm for him.