Learn Portuguese (Portugal): O menino está sempre a tirar a fralda!

This is a core feature of the European Portuguese continuous tense. Short, common adverbs like sempre, ainda, já, and bem frequently slip between the auxiliary verb (estar) and the preposition a. You could technically put it later in the sentence, but tucking sempre in the middle sounds much more natural and fluid to native speakers.

Why does the sentence use a fralda (the diaper) instead of a sua fralda (his diaper)?

In Portuguese, when an item of clothing, a body part, or a personal item obviously belongs to the subject of the sentence, the definite article (a or o) is preferred over a possessive pronoun. Since it is the boy's own diaper he is taking off, a fralda is the standard and most natural choice.

When should I use tirar instead of despir for taking off clothes?

Despir generally means to undress entirely, or to take off a major piece of clothing like a dress or trousers. For accessories, shoes, and specific removable items like a diaper (fralda), tirar (to take off / to remove) is the correct verb.

Would it be acceptable to use the gerund and say está sempre tirando?