Vomiting in the morning only

I was hoping to not have to deal with an ill bird so soon as I'm new to their care but despite my fastidious cleaning of cage surfaces and attentive removal of old food/any bacteria sources like poop, Matisse has become sickly. We have contacted 2 avian vets so far today alone and left messages, waiting to hear from them still. One wasn't working at the clinic today and the other just came back from a vacation on the 15th supposedly according to her voicemail but didn't answer so I left a message. The latter vet mentioned also gave me advice when I started noticing the whole seed vomit in Matisse's sleeping area, she suggested it could be hormonal behavior since he's not showing any other symptoms other than the puking of a solid seed loaf and a little bit of clear liquid after but we weren't able to secure an appointment at the time as she was about to leave on the fore mentioned vacation. 😵‍💫

My main issue is, how do I help until we schedule an appointment for him? So far, I've been even more diligent with cleaning and sanitize the cage bars daily, offering only a single vegetable a day with pellet powder instead of their usual frozen chop to keep meals easier to digest, clean water, and pellets always on offer as well as their daily seed ration on only plain paper towel. I've also been offering Matisse more millet during out of cage time to be sure he is consuming at least something (I do see him eat just fine through the day). Matisse is the only one behaving in this manner, Picasso is completely normal and himself. The strange thing is Matisse seems to puke up that seed, then sit quiet for about 30 - 45 minutes, and then acts completely normal the rest of the day so far as to even try to court Picasso. 🙄 I appreciate any advice you could offer me and I'll be busy still calling around as I've been trying all week to find someone to see him. There's an emergency clinic we can visit but it would be immensely more expensive as they would charge an extra emergency fee and a new patient fee onto the bill (we've taken our rats there before but each individual animal is treated as "new" for the first time they come in). If that is what must be done, I will not hesitate. Thank you for reading my distressed post, I look forward to hearing your thoughts on what I should do next.