Is it legal to get free advice from a friend regarding investing?

Assuming that your son is not employed by a financial institution and your son is also not employed by any firm that he is trading in, then the general answer is that he will not get in trouble with the SEC.

Each state has a de minimis rule. It is always much smaller than the federal rule. A person can provide advice to some number of people without running afoul of state law. That number is usually very small (3). Your son would still have to conform to all aspects of the state and federal securities laws, he just would not be required to register. Federal law is more flexible than state law is. It is far easier to run afoul of state law than federal law in a situation like this.

The IRS is another matter. Any such transaction would have to be structured as an arm's length transaction. The IRS has a variety of penalty taxes that could be triggered if the transactions do not look like they are being operated as if you were strangers that did not trust or like each other. At the minimum, either of you could find yourself paying a gift tax to both the state and federal governments. Other issues could be triggered if the account is a retirement account or an IRA.

As a rule of thumb, if your son said "go buy Apple stock today," and no money changed hands, then nobody would care as long as he had no insider information and did not work for a financial institution. If your money is commingled with his, or, if funds changed hands such as by paying him, then it could trigger legal or regulatory issues. The one other exception to that would be if your son were performing some type of market manipulation, then if you followed his advice, you may be deemed as manipulating the markets as well.

If the money is any sizable amount and you were either considering commingling your accounts or paying your son, it would probably be worth a trip to an accountant. It would also be a good idea for your son to understand the Investment Advisors Act, the two principal Securities Acts, the Investment Company Act, and your state's Blue Sky Laws. Most states have adopted the version provided by the Uniform Law Commission, but there are notable exceptions. If you are in Louisiana because it is a Civil Law state and not a Common Law state, other issues come about as well. If the both of you live in different states, then the laws of both states must be conformed with.