I'm not sure I understand. Your grandson has already chosen his school? If that is so, then is it just that they didn't recruit him? If that's the case, then there would normally be a walk-on tryout process in the fall. He should contact the coach there to express interest and ask about that process. The coach (or an assistant) might then come see him pitch before his season is out.
The description you give matches that of many kids who end up with some scholarship money, but also of many who don't. Lots of things go into that. Being pro-active is no guarantee but it increases your odds a great deal. Conversely, sitting around waiting to be discovered reduces your chances a great deal, though some do indeed get discovered despite not being very pro-active. The bottom line is, many qualified kids do get overlooked, so if you're not being proactive you greatly increase the odds you'll become one of them. And if he hasn't done anything up until now, just six weeks before graduating, he must not have wanted it very badly.