I wish we had that problem with Maddie. She's not even four months now and she climbed a full flight of stairs to our second floor to get to Jaime! That'sll teach me to take my eyes off of her. I wouldn't mind but she's tumbled down the three steps to our front yard more than enough times to make me worry about a 13 step journey
You can try ramps or very small steps. Maybe even stacks of phonebooks to give him an easier way up. Encourage him whenever he makes an attempt. Lots of praise for effort and lots of excitement to try to get him to go. It seems to work for us when Maddie gets a little stage fright.
Tom