you need to give him time. I adopted a 4 year old shepherd about 6 years ago who was terrified of the stairs to our apartment. she could see between them. It took a lot of positive encouragement and not making a big deal out of it. We carried her up a few times which, looking back, didnt help matters. Praise and encourage. If there any particular reason you want him in the basement?
Shasta GSD 5/5/10 CGC, ITD, TC
"Dax" Thor z vom Weberhaus BCAT CAX2 CGCA ETD HCT NCO-1 PKD-T RATI RATN RA TC TKA 3/18/2013
"Hades" Guilty vom Blutfeuer 10/26/2018
Zena GSD 6/1/03-2/16/2016
Riley GSD/BC 1/10/05-2/1/2013