How can i protect my child from infections if he has leukemia'?

Common sense: Use of soap and water or waterless soap by your child and those who touch him goes a long way. Also, reducing contact with non-family members and enforcing that adults and children who know they are ill or feel like they are getting ill stay away or at least where a mask and avoid touching if possible.

Hygiene: Stay away from those who are ill. Wash his hands, your own, and visitor's hands frequently, clean commonly touched object frequently (doorknobs, toys, refrigerator, etc) avoid groups when his counts are down. 'cocoon him' (vaccinate those around him). Eat only well washed fruit/vegie, well cooked meats, pasturized milk. Best wishes to you and your son.