I'd say it's a very good indication but I would not necessarily assume a "roster offer" - I think it just depends on the way the HC and program recruits. I have heard of recruits who were asked to go on an overnight visit but did not end up receiving a "roster spot" or other offer or assistance from the HC. That's just anecdotal, but probably not a bad way to look at it - per Bishop, keep open as many doors as possible. D3 recruiting is very fluid and if there is someone higher on their board at your son's position who comes back into the equation, they may not commit to a spot for him. It just seems like there are as many approaches to D3 baseball recruiting as there are D3 programs. It can be frustrating for sure.
Hope he does get an "offer" that works for him and that the school is a good academic, athletic and social fit!