In Spanish, the adjective "perezoso" (which means "lazy") can be placed either before or after the noun. However, when it comes to more descriptive nuance, the position can affect the emphasis.
In your sentence: "Diego es un chico."
The adjective should be placed after the noun if you want to say "Diego is a lazy boy." Thus, the correct sentence would be:
"Diego es un chico perezoso."
So you would place "perezoso" as follows:
- (1) Diego (2) es (3) un (4) chico (5) perezoso.
The adjective "perezoso" follows the noun "chico."
In summary, the best place to put "perezoso" is after "chico" (section 4).