Domestic shorthair, broken mackerel [TABBY). Color seems to be blue (grey).
Lovely kitten, accepting kids. And the girls are very careful and easy with him -
Im not sure what to say about the collar. If you arent afraid he runs outside and gets lost, its better not to have this... The collar can easily fasten into something. Kittens are adventorous discoverers and climb everywhere....
I agree that this kitten looks like a brown mackerel tabby domestic shorthair. You could also describe the pattern as "broken mackerel" since there are some stripes and some spots visible in the pictures.
In my opinion, it appears to be a brown (black-based) tabby and not a blue tabby, since the stripes are black.
It is a very cute baby, but it does appear quite young. Was there a reason it had to leave its mom and siblings so early? Did you rescue the kitten from a desperate situation?