For sure, exact location of registered sites are not going to be shared by the state of Ma. Nor would I expect it, or encourage it. There are good reasons for this, since some collectors will damage protected, unexcavated sites. I recall years ago a site in one of our state parks in RI was looted during the off-season. Archaeologists returned in the Spring to find the site destroyed.
At the same time, it’s understandable if any collector would be reluctant to share where they find things at sites they are themselves still actively hunting, or plan to hunt in the future. We would not share that info with another collector competitor, nor would most of us share that info with a pro. There are exceptions, and it has to do with building trust between collector and pro.
Recently, a field my wife and I had to ourselves for 30 years, and in which we found hundreds of artifacts, was sold for development. I was very pleased to work with the archaeological survey team doing their thing walking and digging test pits. In no time, I filled them in on what was present at the site, and where they should concentrate their test pits. After walking it for 30 years, I was glad to help properly record and register the site.
The relation of pros to amateurs, and to collectors is often strained, and trust is lacking. It’s likely this pro only wants the info for the very reasons he stated, and the OP will not somehow find himself banned from the site. But caution is very, very understandable. I would get to know this guy well first. It’s not like Brewerton Eared Triangles are at all rare, and it’s not as if Late Archaic sites are rare. The pros know full well that we collectors are out there finding and hunting sitrs far more than they are, and of course if they can learn from us, by asking these questions, they will. I imagine if you had found a fluted point, he would be chomping at the bit to know where you found it.
If archaeology and knowledge of the past is more important to a hunter/collector than finding and keeping the material artifacts of the past, this type of sharing comes far easier. I walk in both camps, and do believe knowledge of the past is more important. But that does not mean I just share all my locations. I usually do not if I am still actively walking them, and, in any event, I learn who to trust, or I give up info when a site is slated for development and the pros have no time to learn everything I learned in 30 years, so I will gladly share my knowledge of such sites, in the interest of advancing knowledge of the prehistory of such a site.