Harness training. Easily the single most difficult thing to teach a parrot, especially if they were not desensitized to one very early on. It took me a whole YEAR, training every night, before Salty would accept his harness without constantly trying to bite it off. If I were you, start first of all getting your Sennie used to you handling him all over his body. Rough housing with him, try lying him in your hand on his back, that kind of thing. If he does put his head thru the head loop thats great! When he does, keep the treats flowing while he has it in there. WHen you attempt to put his wings thru the loops, keep then very large and loose, you can tighten them up after he has it fully on. Praise and reward of course when any movement forward is evidenced.
Be consistent, be immediate and be patient, as with any training.
