I think the term reticulate is usually used to suggest the pattern of the birthmark....sort of a birthmark blotchy pattern that seems to follow several lines. A reticulate pattern can mimic a vascular condition called CMTC. There are tests that can determine if there is truly CMTC. I believe a simple test that shows one of the possible markers for CMTC is whether it changes color with cold or warmth. But that's best left to the doctors to determine.
I've also heard mention of a vascular mark called a reticulate naevi that is closely related to a PWS...but it is suggested that it actually fades over time. Though I've only recently heard of this....not sure of the reliability of this info. I'd do a google i guess.
As for laser treatment....it's a personal decision. Generally, the younger the skin, the easier it is to see results. Limbs are notoriously stubborn. But if the PWS is not typical, there may be some reasons to not jump into it. If it covers a large portion of the arm, it would most likely require general anesthesia, and that would also be a reason to wait a bit and let him gain some weight, get older, etc. so that he'll be better prepared to have less chance of any side effect from the GA. 6-12 months is a common age. We started at 15 months...but we also didn't have a definite diagnosis until 12 months. Waiting for 6 months in your case may be a good idea....just to make sure everyone is on the same page with any treatment plan or diagnosis.
Most often there is no question that you're dealing with a PWS, but "I'm here to testify!!" ;D ;D, that not everyone's birthmark appears in the typical fashion. Our son most definitely has a PWS....but we could no see it when he was born. His actually progressed in color over time.
That there is no suggestion of KT is awesome!!!
Hope all of this rambling didn't make you nod off...