In formal English this sentence would be: He eats more slowly than you do. (Slow is an adjective that is used to modify a noun. Slowly is an adverb that modifies the verb. Adverbs say how someone does something. The word "fast" is both an adjective and an adverb. i.e. His speech is fast, He speaks fast.)
it looks like धिरे is invariant- i.e. doesn't change by gender or number, the same way e.g. in English we have chicken/chickens dog/dogs but sheep/sheep. So here they're both masculine singular, despite appearances.