Disclaimer: I have no experience with those particular states.

From what I have seen, generally ALL income earned in a state is taxed by that state. Then some amount of a credit is received in the Resident State for tax payed to another state.

With most states I have seen,,,, he should pay tax on all of his NY income in NY. I think your NJ allocation makes sense for the income to report in NJ, but there likely is a credit for the portion of the tax paid in NY on that income on the NJ return.