Taking up family business and responsibility at 18 can be overwhelming. It requires a certain level of maturity and grace to handle. I'd advice is to stay close to his mom, as she will always be there to advise and support him. She wouldn't wish bad for him or want her late husband's legacy business to crumble.
Secondly, getting a financial advisor is crucial. If he doesn't trust his dad's lawyer, a financial advisor can educate him on managing finances, investing, and growing the business. This might help him develop a passion for the business.
Since he's young, he might not be able to handle everything, he should stay in constant communication with his mom and get a financial advisor. Together, they can find a manager to run the business until he's old enough to decide whether to take over full-time or continue with his path. Having a financial advisor is very important in this situation.
Taking up a family business even at 40 is very difficult, things aren't as easy as it looks from outside, too many legal stuff happens that causes too many economical decisions and if you make even one of them wrong, you take a step back for months, and in this economy it's very hard to recover from that.
I agree with financial advisor, pick a good one though, not some useless one looking to get paid, a professional that has a proven record is a must, when you are that young, it's very hard to admit that you don't know anything, most people at that age will think they know it all, so being humble is important while still having a lookout for whoever is helping you. On the long run, we can't really consider this as a good situation to be in.