Why is my dog still having accidents inside??

For context, we adopted our Cavalier pup back in July. By that point he was 6-7 months old and came from a rough start. He was found in a hoarding puppy mill situation but considering all of that he is healthy and so happy! He’s now about 14 months, and while he has improved so much with potty training, he is still having (pee) accidents. Sometimes he goes a few days without, but days like today he did 2 pees inside. He’s really good with going poop outside so I take that as a win!!

I’m wondering if

1) he’s still adjusting/learning considering he came from a hoarder and probably just went potty anywhere.

2) he’s my first small dog (14 pounds), I’d like to think that’s harder to potty train than bigger pups?

3) is there any reason I should have a health concern? being that he only poops outside just fine.

4) anyone else experience something similar? If so, any advice is appreciated

THANKS!!