"The child is eating his candy."
Translation:הילד אוכל את הממתק שלו.
You need to drop the article when you say "ממתקו". So It's either "הממתק שלו" or "ממתקו". The suffix makes the noun definite which means you don't add the definite article (ה). By the way, I rarely hear native speakers use the "ממתקו" form. There seem to be certain words and phrases where the suffix is used -- usually in more formal settings. Certainly not discussing candy :) .