Start from the beginning, have you wormed him? is he penned with other calves or by himself? When you say grain and sweet show feed mix what exactly is that. How much protein to fat is in the feed, is it a Starter, grower developer or finisher? He is eating more than 3% of his body weight at this time, so appetite is not the problem.
At his weight this is what I would do, make sure the feed he is getting is what he needs at this stage, I would make sure I fed at the same time each day, 6am and 6 pm or close to that (you need to set a schedule and stick to it, calves tend to do better when they or on a strict feed schedule, less stress if they know they are going to eat at a certain time every day). He needs hay, quality grass hay along with his feed to keep his digestion system on track, Feed the grain first and once he cleans it up feed the hay.I would drop him down to 18 pounds (9 morning and 9 at night) and see if he does not clean it up more efficiently.
Every calf is different, managing their feed and staying on top of them is the difference in finishing a calf at the right time or playing catch up throughout your project, He needs a feed to get him to grow right now, and filler is not going to help you right now, a balanced developer to maximise growth and weight gain is what I would be feeding him a complete feed with a protein level around 12.5-14.2 and fat around 2.9-3.5, many of the pre mixed show feeds should meet those needs. I like to find a brand that has feed for the different stages of growth, changing them over is easier and the feed is usually similar in type and texture.
Do not panic yet you have time, your goal is a 3 pound average, I have had calves gain 2.5 for a while and then hit a growth spurt and average 4.5 for the next month or so.