3and2, if your post had not mentioned Milb aspirations for your nephew, this thread might read in a very different way in terms of going where you are loved (D3).
Our son, as a freshman in HS, was quite similar to your nephew in size and 60 time. As a senior, the size and 60 time were also similar.
Coming out of HS, our son had one D3 coach who loved him. That D3 coach saw the potential for him to be drafted out of college and was quite open about that potential and the process to get there.
That D3 coach was also very vocal with his D1 coaching friends about our son. During most of our son's HS senior year, D1 coaches found reasons our son would not succeed (mostly his weight and strength). At the end of April, our son deposited at the D3 which had recruited him so religiously from the prior Fall. About that time, a number of local D1's came to see him and his Legion coach called nearly every D1 in Northern CA telling them they were crazy in overlooking our son (sound familiar.)
This brought the local JC and a number of coaches to his games with offers coming even as late as July and early August (mostly walk-on) with one confirming a scholarship offer would be made at a campus visit (this was about 10 days before school started).
The D3 our son attended was the same one referenced by BOF. That school is in the most scouted area of the Country. The D3 college coach had our son in front of scouts from their very first Fall scrimmage and they continued to follow him each step of the way (with one indicating he would be drafted as a position player after his junior year-he was not). Our son was placed in the NECBL with top teams in that league, played well against Team USA during 2 separate Summers and made that leagues All Star team twice. Along the way he had options to transfer to one of the very best D2's (from FLA.) and also transfer to D1 programs including one in the ACC.
After graduating in 4 years, our son was drafted and in his second year he was voted to the MWL All-Star team. After an injury cut his Milb career short, he went on to coach at the D3 before moving back to CA and coaching at the D1 level and being quite successful as the hitting coach at different programs.
More than a few from this site and elsewhere have said our son is an exception in terms of D3 position players being very successful in college and Summer Wood bat leagues and being drafted.
It certainly is not as easy as it might have been at a top 100 D1 program or the very top D2 in FLA. It does happen. though. I tend to think you need the following:
1.) a top D3 program and coaching staff;
2) A coaching staff which is linked with and respected by the scouting community and which is tenacious in getting visibility for their players;
3.) Having at least one plus tool (our son's included his arm, athleticism and Consultant's 6th tool)
4.) getting better every single year in terms bat speed, range, etc and strength and explosiveness;
5.) playing against top Summer league competition and succeeding at a high level;
6.) having a ferocious mental attitude and toughness
Good luck to your nephew in his HS senior year of baseball and working through his choices and options.