Hi J.,
No you didn't wait too long. :) Although it may feel like it! One thing to note before I give you my suggestions: It should be someone other than you giving him the bottle for now...he will look at you like you are nuts trying to give him his milkies from that plastic thing! LOL!
The bottle I've had the best luck with for not only situations like yours but also therapeutically to bring nipple confused babies BACK to the breast is the BreastFlow Bottle by First Years. It actually is designed to mimic how babies suckle at breast by requiring compression and suction to get their milk. It isn't identical to breastfeeding, but it is closer.
Regular bottle nipples are firm and shaped much differently than the breast so of course baby has no clue what to do with them. :) The BreastFlow fits in the mouth differently and, in my professional opinion, is much more effective than the Adiri. Having tried both with my clients (I am a Lactation Consultant and specialize in feeding issues as well as work with the basics of latch and positioning) I much prefer the BreastFlow and I am not one to really promote one thing or another...I use what works best for each individual baby.
The research is showing us now that orthodontic nipples are not ideal for babies as it causes them to suck oddly using more of a tongue thrust motion (pacifiers too) to get their milk (imagine that on your nipple!) so those of us up on the research do not recommend those at all. Avent is just too firm and the nipple is short...wide based does not necessarily mean the nipple is a good one...
So - give the BreastFlow bottle a try. I was super skeptical about them at first but used one out of desperation with a baby who wasn't able to breastfeed well at all but was refusing his supplementary bottle in favor of the breast (not a safe thing for him at all!). His momma happened to have a BreastFlow so we gave it a try. This was a kid I wasn't sure would ever be able to breastfeed 100% because of his issues - but not only did he take the bottle just fine (because it is so much like the breast - soft and squishy and fills the mouth similarly as the breast) but he also figured out how to do on the breast what he was doing on the bottle - the consistency of suckling motions was what did it for him and he is now breastfeeding 100% and doing great! Since then (this was a year ago almost now) I have used this bottle with many babies with the same type of success...
Good luck! Hang in there - keep trying and you will get there!
Warmly,
J. Simpson, IBCLC, CIIM