Well now...
If you want him off the breast by 9mo, your only option is to give formula, if you don't want to express. Cows milk is too high in sodium at that age to give as a BM substitute. You can give formula in a doidy cup - there is no law which states it has to be given in a bottle! I think you've got zero chance of him taking a bottle now and he's old enough to know it's no substitute for a boob, so I think that option of a 'mum-substitute' is gone.
I went back to work FT when my first was 8mo and a complete bottle-refuser and wouldn't even take expressed milk (frozen or freshly-squeezed) from a cup of any kind. He also ate next to nothing so I was beyond sick with worry about going back to work. However, without me around, he actually ate like a horse at nursery. I just got them to mix expressed milk into his porridge and give him an extra yoghurt or two to make up for the lost milk feeds.
However (again) he was still feeding every 90mins to 2hrs at night at this time, and continued to do so until well past a year, so I knew, if nothing else, his milk requirements were being met at night. Any attempts to reduce his night feeds were met with screaming, screaming, screaming...I even left DH to deal with him for a couple of nights and slept with ear plugs in. DH couldn't hack a third night (I was well-hardened to the sleep deprivation by this stage, which he wasn't) so it was back to being the all-night milk bar until he was developmentally ready to sleep more independently. I tried all other methods of sleep-training as well to no avail.
Do bear in mind, though, that night breastfeeding is about a great deal more than the milk which is why babies do not like parting with it- it's reassurance that primary caregiver is still there and they're not going to get eaten by wolves, it's comfort after a bad dream, it's relief from teething pain or any other growing pain, it's relief from boredom in a dark room...the milk is a bonus and why introduction of solids may have zero influence on the sleep of a breastfed baby. The ability to not breastfeed at night is, as I alluded to earlier, a developmental skill, and has less and less to do with nutritional needs as they get older, but at 6-9mo, nutrition does still play a big part. After my experience with DS1, I didn't even attempt to night-wean until after 12mo with DS2 as I knew his nutritional needs were definitely being met in other ways and he was more likely to be developmentally ready to not need boob-support to get back to sleep at night. Basically, when you do it, lie with him and have him scream in your face and pull your hair for three or four nights until he knows night boob is no longer an option. People talk about Jay Gordon's 'gentle' night weaning - that's basically what it amounts to. I think it has to be done cold-turkey as well - JG advocates no feeds until midnight, then feed thereafter, and gradually extend that time, but a baby can't tell time and I think that's just confusing and prolongs the process. Do it cold turkey and get it done, but again he doesn't advocate doing it until after 12 months.
In short, whoever you leave N with when you have to go away will need to be prepared for a rough couple of nights, unless you're able to a) change your plans and not go away or b) physically remove yourself entirely from N for 4-5 days and leave your DP with a bottle and see if N will take this in desperation.
Sorry. No easy way around it!