No problem giving other supplements. And you don't have to give the Microlactin with food. It won't upset the stomach. Some dogs will just eat the pills like a treat. It kind of smells like vanilla or malt. Chance is just old and picky.
Is Moose on any pain meds? (It won't interfere or negatively interact with those either...I'm just curious)
When you switch to Orijen do it gradually. Start w/ 1/4 c of Orijen mixed with ProPlan, then go to 1/2 cup then 3/4 then entirely. Do this over about a week. Otherwise you MIGHT have an upset tummy. Origen is more rich than Pro Plan. Also...you won't need to feed as much. It's higher protein...so watch his weight!
Oh and I just went and read your thread about Moose. I'm so sorry you're both going through this, but he might surprise you and with care and pain management, he may be around for a LONG time, and have a comfortable life.
I didn't read the whole thread but have you tried acupuncture? My Mira has spondylosis...from being such an active crazy youngster. She'd jump out of windows (take the screen with her...first floor thank heaven), over 6' fences from a standstill, would dig UNDER said fences and shove the metal up with her back so she could wiggle out, etc. She was a wild child. When she got spayed, being on that metal table almost did her in. Along with the back issues caused from the spondylosis and the metal table, she was getting infections everywhere. She was in SEVERE pain for a month. Could barely walk..her legs wouldn't support her. It appears the spine was compressing nerves. She wasn't eating and lost 15 lbs...and she was already thin. Infections weren't going away. It was awful. Nothing worked to fix it...until acupuncture.
She didn't change drastically after the first treatment, but I noticed she was moving a bit more, seemed much more comfortable and started eating. Then after the second treatment...she was running. From that point on she just got better. The infections went away (yes she still got antibiotics too) and the back was feeling good again. She was back to her active crazy self...but the spondylosis wasn't "cured" ... the pain was gone as was the numbness in her legs. That's what acupuncture does....it stops the pain mechanism somehow.
She had 5 treatments total. Once a week for 4 weeks then one a month later. The change was amazing. And another of our dogs, Lucy, had the same reaction (even better - she was running and jumping after the first treatment and she couldn't even walk into the Vet when we got there) when she hurt her back.
Mira will now have pain in her back about once or twice a year, and a two day regimen of a NSAID from the Vet fixes her right up.
It's been 4 years since then...and she's doing great. She was SUCH a mess, they weren't sure they could save her. So keep your chin up. You never know. And please TRY acupuncture if you haven't yet, it's worth a shot. It won't "fix" his problems, but it may well stop him from hurting.
Sending lots of good thoughts for both you and Moose.