What is the meaning of "he couldn't help noticing" here?

“He couldn't help noticing” is a shortening of the phrase “He could not help but to notice”, or “He could not help but notice. These are a form of the phrase, “he noticed”. But, the aforementioned phrases place a special sort of emphasis on the striking nature of the notice.

For example:
I notice you cut yourself shaving.
Vs.
I couldn’t help but notice that someone slit the dead man’s throat.

In the case of the clothing that was noticed, it was so peculiar that it was glaringly different to what he was accustomed.

Getups is synonymous to costumes. Particularly strange or out of the ordinary clothing may be considered a getup.

I couldn’t help but notice the clown getup you wore to the job interview for the law office.