HUDSON — Unfortunately, it’s back to square one for Brenda and Ken Grabast. A life-changing stem-cell treatment for their son has been canceled with no indication that it would be rescheduled at the XCell-Center in Dusseldorf, Germany.Brenda Grabast has received an e-mail confirming that the XCell-Center has shut down its stem-cell treatments until further notice.“The final sad message is that we cannot offer stem cell therapy anymore — maybe in the future — we do not know yet,” the message read. “Therefore and very unfortunately, I have to cancel your planned treatment.”Derek Grabast, 8, was to undergo a stem-cell procedure to treat non-verbal autism and spastic quadriplegic cerebral palsy.