Not enough information for a qualified answer. I will make some assumptions and spell them out. Basic assumption: You actually care about being DDoS'ed (you earn money doing live streams or something).
First, I assume he means a static server IP address, not your home IP. Most ISPs use dynamic pools to assign IPs and you will get a different IP from time to time - sometimes every day, sometimes whenever you reconnect, depends on your ISP. If he means your home IP, you are good, because in 4 months you most likely have a completely different IP now.
Then again, a static server IP makes no sense to "find out". They're public. All it takes to "find them out" is "ping domain.ext" and there it is. Which also means that changing the IP doesn't do squat.
Second, I assume that he actually has the capability to make a DDoS attack of sufficient strength to knock your server offline. Most likely that will trigger the anti-DDoS protection of your hosting provider. It is still likely that your server will go offline. It is not likely that it will stay offline for long, as your hosting provider will take action - they don't like DDoS traffic clogging up their network.
Third, I assume that he doesn't have sufficient capability to knock your hosting provider offline. People who sit on that kind of botnets don't generally use them to threaten random people into "re-friending" them, that's just ridiculous. You can make real money if you have a proper botnet. He'd rather do that.
Possibility a) we're talking about a server and a person who has the actual capability to run a DDoS. Advise: ignore him - sure he can knock you offline for a bit, but it'll cost him more than you and it won't last.
Possibility b) we're talking about your home IP. Then either - b1) the guy is a troll who's trying to scare you and he actually hasn't half a clue about how things work. You've most likely changed IP since then. ignore him, he won't harm you. Or b2) he keeps track of your changing IP and actually is only half an idiot. He could DDoS you and disconnect you, which might be shit during a game or a live stream. If so, talk to your ISP and report him to the police (threatening negative, illegal consequences if you don't comply with a demand is a crime). Also report his friend as an accomplice. Note that, depending on your jurisdiction, two people collaborating to commit a crime is sometimes enough to qualify the deed as organized crime - I'm not kidding. Once these people get involved, the shit hits the fan. Do not inform them that you've reported them. That could be construed as interfering with a police investigation.
Involving the police might seem harsh. I don't know the circumstances of your case. If you think "wait, no, that's not what I meant" then my basic assumption may be wrong and you will not be seriously affected by a potential DDoS. If the whole thing is mostly inconvenient and just scares you a bit, then ignore him and if he does eventually DDoS you, disconnect, read a book for an hour, come back and you probably got a new IP from your ISP and are online again. If not, call the ISP technical department and complain about a service outage. They'll figure it out, filter the DDoS traffic and put you online again.