iMessage does not work with one contact after switching mobile carriers.

So this morning I switched from Verizon to Metro, and I kept my old number. The transition went smooth and I was out in 15 minutes. iMessage works just fine with everyone i usually message, except for my brother. He has an iPhone 12 and iMessage is enabled on his device, but our text conversation is SMS only. We've both restarted our devices and made sure iMessage was enabled. I do have two phone numbers, one of which is managed externally for my work, but he only has my personal phone number in his contacts, though occasionally messages send from my personal cell phone using my work number which is bizarre in its own right. The reason I'm wondering about this is that the primary reason he switched from Android to iPhone is for the features that iMessage and FaceTime provide, but if they don't work, what's the point, you know? but anyway, I tried googling it but apparently my question is too specific and I wasn't able to find anything via support forums here. let me know what I should do. Thank you in advance!

Tech Specs:

Me:

iPhone 13

iOS 15.5

MetroPCS

Brother:

iPhone 12

iOS 15.5

Verizon