If your ex hasn’t seen his son for 7 years, it seems likely he doesn’t want to be in his life. If he doesn’t even pay maintenance, it is possible his new wife is unaware of your son’s existence.
If you approach him now it may be a big shock. I’d do it discreetly, allow him time to get used to the idea, and give him time to break the news to his wife. Be prepared he may refuse to see your son.
Don’t tell your son before you try. I’d wait and see how he reacts. Good luck.