Hi & thanks for using our service. I'll do my best to give you a complete & accurate answer. Please ask me to clarify anything you don't understand.
Chances are, since the trust is irrevocable, it may have been required to file income tax returns in the past. Who are the trustees? Was the stock sold & then the proceeds distributed to your son? Or was the stock actually distributed to him & then sold? It will be necessary to determine the original number of shares contributed to the trust and their cost when your parents purchased them or the number originally purchased by the trust and their acquisition cost; as well as the cost of the shares purchased through the reinvested dividends in order to determine the adjusted cost basis for determining gain or loss. The dividend reinvestment statements will have the latter and your father's old company should have information as to his stock purchases if made as part of an employee stock purchase plan. If he purchased them through a broker or a bank, then that company should have similar records.
I suggest that you might consider getting some professional help, possibly from a local CPA, to make these computations and /or estimates with/for you, if you aren't familiar with these issues, as we have several different sources of data, that we use to obtain reasonable estimates if the actual records are not available.
It is really beyond the scope of what we can do here in terms of trying to give you specific details as we would need a lot more back & forth in order to determine the specific facts.