My boyfriend has one ball the other one he had remove as a baby cause he had cancer. .....When he ejaculate it's very little why? Can he have kids?

FERTILITY. When in doubt see a urologist. Yes, you can get pregnant.
Not sure and yes. Volume is more from prostate and seminal vesicles, only about 5% from testes. Removal decreases fertility by about 10% but the other treatments (chemo/radiation/surgery may worsen that.
He needs evaluation. When someone has a childhood history such as his, there's an increased risk for future fertility problems. He should be seen by a qualified urologist who specializes in male fertility.