This is normal... it is just kids. They do that.
My daughter, when in Preschool at age 4... had a 'crush' on a boy.... Tommy. She'd blush and giggle when he walked by and then would point him out to me. And then we would talk... to him and his Mommy and she'd just stand there... all blank just blushing and smiley to him. His Mom knew... she had a crush on him. It was so obvious. It was cute.
Her Teacher said... THIS happens, its normal, is benign.
In Kinder as well.
It is NOT a big deal.
In Kinder, a boy, had a crush on my Daughter... they become real good friends. His Mom would laugh and say "at least my son is monogamous... each year, he has ONE girl, that he likes....and says is his girlfriend." To this day... he and my daughter, are GOOD friends. They look out for each other. They are good buddies. Good kids. I am good friends with his Mom. It is all very normal and nice.
Another kid....probably used the word "booty bumped" on your son. They do learn slang... at school by other kids.
If anything, you can mention it to the Teacher... because... Teachers do, as a class... will talk to the kids about 'manners' and what is polite/proper/not allowed. If it is a problem.
all the best,