2/3 of a cup so ends up being 2 cups total he eats per day
Thanks for the clarification. I have a few additional thoughts. Trying to include them all in one post to make it easier, while trying to avoid repetitions:
1. As Boxergirl said, I don‘t think you would cause any harm by adding toppers as long everything stays nutritionally balanced (especially for a growing giant breed puppy). Watching the calcium and phosphorus ratios is probably key.
2. I‘d suggest changing his "main" food first before experimenting with toppers. In case he really doesn‘t like his main food too much, even the best toppers won‘t do much in the long-term.
3. It‘s probably a good idea to formally track growth. I used a simple Excel sheet including a table and line chart. x-axis = time (weeks), y-axis = weight and/or height. You should probably see somewhat of a straight line while they are young and growing fast. It doesn‘t have to scientifically exact/perfect, but can help with identifying certain trends or to answer questions the vet or breeder may have.
4. Are Mastiffs generally picky eaters? I don‘t have the data to proof it, however, my EM is probably one of the pickiest eaters ever. I‘ve heard similar story from other Mastiff owners, so it may be a thing.
5. Who is slow at growing today can be very fast at growing tomorrow. They just go through so many cycles in development. Some Mastiffs seem to be gaining most of their height and weight in the first year, while others take much longer.
6. Enjoy the time with him. I know finding and choosing the right food can be stressful. When in doubt, you definitely want him to be on the leaner side (maybe that‘s what his genetic body type is anyway). Nothing worse than an overweight, growing giant breed puppy.