As I said in my previous post, which happened to take up a good 10 minutes of my study time so I'd appreciate it if you read it after I defended you also. Please don't ride until you have your saddle fitted, and don't get on without a mounting block.
yes your friend holds the other stirrup of their saddle when you get on their horse to stop the saddle being wrenched across the horse's back thus inflicting spinal, musculature and nervous damage to the horse. It IS a good idea to hold the stirrup. You have all the answers you need from this thread. Now go and use them and if they don't work, THEN come back and ask again with more specific details and video if possible.
Again, as I said previously, I would put my money on it that your saddle is causing him discomfort, and when you get on their enhances his discomfort by dragging the saddle across his back. You don't want to get a young pony into the habit of rearing out of pain when you get on, as it may well become firmly established in his mind set. So think of your pony's welfare, well being whatever you want to call it, and do NOT ride him until you have your saddle correctly fitted, and invest in a mounting block/crate of some description.