Keep in mind that this is a business and the goal is to make money for both parties involved. The advisor/agent develops relationships with players based on a couple of different things. Some are eager to get a part of the signing bonus, then they lay low. Others, especially the better ones, from the bigger agencies, are more interested in whether the player will someday sign a larger contract, that is where the agent makes his money on the player.
A couple of questions. Where was your son drafted (which round)? Is this an advisor or agent? You said he had 16 ML guys. Was this a favor and did he receive money from his signing bonus. Also keep in mind that he may not have been able to get your son a deal, based on his draft round. Companies look at it as an investment and want to make sure that they get a return on their investment.
Most guys drafted low do not have agents, until they prove they belong, then everyone wants to buy them dinner.
There are all types of agents out there, I always tell people buyer beware. My son was fortunate to develop a relationship with his agent that if he needed him he was always there, that included phone calls, visits to see him pitch, texts after good/bad outings and equipment and card deals after he was drafted. Also because son has had injury issues, he always took care of everything. No matter what happens they will always be friends. That isn't unusual once the right guys pair up.
If you feel your son deserves better, than look for someone else, but keep in mind agents take on players because they feel they will someday soon be a ML player. If this guy took money from your sons bonus and has done nothing for him, you need to make a call, that's totally unfair and frankly unethical, IMO.