Unfortunately, I do not have good news for your brother. Warrants are always bad news situations and they never expire. You can't "wait out" a warrant or run away from it. It will remain out there until your brother comes back before the judge who issued it and surrenders -- voluntarily or involuntarily -- to deal with his violation of probation or for the rest of his life, whichever comes first.
A criminal defendant gets no credit for time out on the street when he's on probation. As he still owes probation time, when he returns his probation would be revoked and the judge could resentence him to anything on up to the very maximum amount of time that the statute would allow for. He doesn't have to give him credit for time on probation.
If he had served all but two months of his five-year probation when he absconded, it is possible that the judge would be somewhat lenient on resentencing, particularly if in the 18 years he's been gone he has not had any criminal convictions and can show evidence that he has turned his whole life around. In my experience, probation will be asking for years rather than months and the judge would be inclined to grant that unless his accomplishments have been exceptional or that there were other special circumstances.
In any case, he doesn't make it any better for himself by not turning himself in. There are benefits and entitlements he may some day need which are not available to people with outstanding criminal warrants. It is always better to come in voluntarily than to wait for the police to find him and bring him back in handcuffs. If he's going to do this, however, he should not do so without first hiring a lawyer to pave the way for his safest possible return.
I'm sorry to be the bearer of bad news and ask you kindly not to shoot the messenger. This is the reality of being a fugitive from justice, and it can't be candy-coated.