It doesn't matter whether they think your child should or shouldn't be eating x, y, z.
It doesn't matter if we think you're right, or if we think a bit won't hurt.
The point is, you are his mother.
You make these decisions.
If it is the choice of you and your partner to do things a certain way, then you have the right to expect that family members will respect that.
Now what can you do if they won't listen to you?
Stop leaving him with them.
If you go to them with him, and they give hime something, remove it at once and tell them thank you, but we do not allow him to have this at the moment. don't be drawn any further, just keep saying that is your decision and you expect them to respect that.
It is down to you to be assertive. If you allow yourself to be walked over, then there's nothing anyone can do to help you.
I guess you have to choose what is more important to you, because taking a stand and physically preventing them from doing this will probably end in a big row, or in not speaking for a while. If you feel that is a price worth paying because of the strength of your feeling, then you have to be strong and put your foot down. OTOH, you may decide that it is not worth having a row over, and you will just try to stop it as much as you can in a subtle way, and make up for it with uber-healthy food at all other times.
You and your other half need to decide what's the best way forward.
But above all, remember this...YOU are his parents.