There are many, many threads (some quite recent) on every facet of this issue.
On one extreme are kids who will go to college, no matter what.
On the other extreme, there are kids who will go pro if drafted - no matter what. (I knew a kid who took 35k and was cut the very next year.)
I just did an analysis for a PAC-12 coach to be included in presentations to the drafted portion (or likely draftable) of the recruiting class. Money-wise, if a kid develops in college as expected of a potential HS draftee [i.e., he will be a high single digit round], the monetary and MiLB trajectory/difference is surprisingly small. And, the potential problems if a HS kid quits during the life of his initial contract (I believe 6 years) include, but are not limited to: refunding a bonus (prorated) he's already paid taxes on and losing the MLB scholarship. In all events, he loses his college eligibility.
There's no harm in attending the requested workouts - if the player feels like it; it just adds to his experience base (my son did several and they were pretty cool; the workouts, however, were closer to the draft and fit nicely with his conditioning.)
Someone posted: "The same scouts will be around in three years. They want to get to know him and will follow him through college." It's a bit more nuanced than that straight line. IF the player goes to a college in the scout's area, the scout will continue to follow him; if the kid goes to school outside the area, another area scout will scout him. There is generally little to no interaction between a club's various area scouts (although all films and reports from whatever source and time will be in the club's database for viewing/reading). So, basically it's like starting over with a brand new person (scout). (Another nuance: many times scouts are deployed outside their home area during the summer - think summer collegiate leagues - and report on players seen during that assignment.)
One minor, transient benefit of being drafted and not signed is the ability to put it on your resume so people who simply don't understand the draft system are impressed. (In other words, baseball people aren't effected one way or the other; friends and peers are impressed.)
It's flattering and confirmatory to be scouted in HS. The system is designed to excite and overwhelm a player and the family. The best approach is to understand what is happening; what are the advantages and disadvantages of either option. An advisor may or may not have your kid's best interest in mind; every advisor brings his/her bias to the table and, therefore, it is up to the family to do the research and in-depth analysis which is then analyzed according to the player's unique position in life. SO, DIG OUT THE PAST THREADS and determine his particular, unique variables.
(I know HS draftees who are in MLB, I know a few who rocketed to MLB for a brief cup of coffee and were cut 5 years after getting drafted, I know kids who were released after a year or two of MiLB, and I know a few who are now 8 - 10 years in MiLB with and without wives and kids. I even know one who was drafted in 2010 who got his degree while playing. There is no predicting how it comes out, except for the lack of a degree or progress towards one for the overwhelming majority.)
Imo, the low risk decision is go to college. A kid - and EVERY HS DRAFTEE IS A KID - gives up alot to play out of HS and has a totally different life trajectory than one who goes to college, and I'm not sure the HS draftee gets all that much.
Pro-ball is a special kind of hell for HS kids.