If Spike has been eating Mmilennia and done OK on it previously, I'd just go real slow reintroducing it. We usually take between one and two weeks to make a switch or to add something new.
Actually, I think that Timerwolf is the company that has a lower protein grainless kibble. We were talking about this earlier today in this forum but in a different thread. I know that one of the companies that Whole Dog Journal likes has a kibble with lower protein, but IIRC, it's still in the 30% range. Puppies should be in the 24% - 28% range, I think... gmacleod has some excellent posts re: puppy nutrition. You can search her posts or just "puppy food" to find them.
Another alternative for you might be one of the frozen premixes. That's basically a premixed BARF-type food and there are several brands that WDJ has listed as one of their top foods. In most cases, all you do is warm it with water. Some people microwave it but I don't nuke anything that I feed my dogs. I've never used this so I can't make any recommendations regarding feeding it to a puppy, but if the protein levels are within the acceptable range for puppies, I don't see why it wouldn't be a good food for Spike to try.
If you like the brand, Solid Gold has other foods that WDJ rates pretty high besides the Mmilennia. I'm feeding Hund N Flocken now to our Lab/GSD mix and he's done well on it. He needed to drop a few pounds and it has helped him do that. I know two Labs that do well on their Holistique Blenz (sp?). Both of those are too low in protein for puppies, though.
Finding the right food(s) for our dogs can be frustrating. What's good for one may not be well tolerated by another. Fortunately, there are quite a few more good choices now than there used to be.