After a re-evaluation I got an invitation to develop a new IEP (even though he already has an IEP). Is it technically a new IEP?

It might either be an amended IEP or a new IEP.  If the re-evaluation was done because the annual IEP meeting is coming up, the IEP developed at the annual IEP meeting would be a "new" IEP and have new start and stop dates based on when the meeting is held.  Even though an IEP is in place, schools often refer to it as the "new" IEP; but there is no gap or loss of services between IEP's - special education services continue seamlessly regardless of a new IEP document being developed.

If it was not done pursuant to the annual meeting, it could be either an amended IEP or a new IEP, perhaps depending on how many changes the re-evaluation causes the team to make to the IEP document.  If just a few simple ones, it would probably just be an amended IEP.  If significant changes are made and the timing is close to the annual IEP date, the team could decide to go ahead and draft a new IEP.  The difference would be that the amended IEP would most likely have the same start and stop dates as the current IEP (unless those were also amended), and a new IEP would have start and stop dates based on the date the new IEP was implemented.

After a re-evaluation (or any evaluation) is done, the IEP team has to meet to go over the results and make changes to the IEP, if warranted by the new data.

I think that, technically, this is a gray area.  It's more of a renewal when a child already has an IEP.  If they found a new area of need, there could be goals and services that were never before in an IEP your child had.  I think this is the language that's standard after a reeval is done.  (Is this a box that was checked on the IEP meeting invitation?)