Spartans organization has an automatic E9 bid, but has opted out at the 2004 level the last two years. Last year they were placed in the BHL National Division after parity. This will be a major step down from an EHF Elite 05 team, IMO.
I would ask them what their intentions are for next year, and if they intend to play E9 I would attempt to verify that they've made adequate improvements to be able to compete in that division.
MY son is an 05 and played up a year and against the Spartan team last season. The Spartan 04 team was not even the second or third best team in the BHL. Not being critical of that team. Seems like they are well coached but I would not leave the EHF 05 for the E9/BHL unless it is with one of the teams that has an automatic bid into the E9, has excellent coaching and your son has the physical ability to play against bigger stronger kids.