I agree with Hortensia (I think it was her) about something he can cherish for many decades - especially since there is a cult between you.
Can you attend? If so, take a few photos of the two of you together and have the best one framed. Give him copies of the others.
I don't know your financial situation, but I planned a few years ahead of time and paid for a nice vacation. In my case the caveat was that it was a trip for just the two of us.
What does he enjoy? What does he plan to do now that he's out of high school?? Maybe splurge just a little and get something nice that will be useful in that activity.
Will he be moving out soon?? If so, that opens a whole other list of possibilities.
A little cash is nice but I wouldn't make the entire gift be just cash. That's like phoning it in and is cold and impersonal. Not knowing your financial situation, I cannot make a suggestion on a cash gift but personally I really think you need to invest more in something tangible that he can hold on to when the WT game starts to unravel on him.
Cash may be spent on video games, fast food or a new JW suit for service - wasted money and soon forgotten in my opinion.
LapTop or Cell Phone are a good option but might be obsolete well before he hits 25.
Congrats to the Proud Papa!