Thanks for the additional history. It certainly helps. I'm glad to hear that he got colostrum. The amount you fed should have been sufficient. From your description above, the tongue travel and drooling are slightly improved, but have not resolved entirely.
I would not feed grain at all to a calf that young. Some calves will show a small interest in roughage like grass hay at early ages -- taking nibbles. I'm not suggesting that you feed it in any significant amount, just that you place a small amount in a feeder so that if he has interest, it's available. Since he has fresh forage available, this may be unnecessary.
When approaching a case, I try to see if I can make all of the clinical signs fit one disease -- usually animals don't have numerous concurrent diseases. The tongue travel is a unique sign combined with the fever. That's why I hit upon listeriosis.
The scours could be due to cryptosporidium (usually found in dairy cattle, but I've seen it in young beef breeds on occasion)or other intestinal parasites like coccidia. Both of these parasites produce mucoid diarrhea. Mucous is from increased residence time of fecal material in the rectum. The goblet cells produced mucus at a constant rate, and if transit rate of feces is decreased, then a larger than normal mucus pellicle will develop around the fecal matter. If there is no stool, mucus is formed, and deposited directly in the rectal lumen. So, in a nutshell, the mucous indicates an issue in the large bowel or colon/rectum.
Is there any evidence of coughing in your calf? Bovine Viral Diarrhea or BVD can cause mucoid diarrhea, fever, and excessive salivation without any evidence of difficulty breathing. You may see ocular (eye) changes with this disease as well like "blue eye". Another possibility is "enteritis" which is caused by Salmonella typhimurium or S. dublin. Clinical Signs of this disease include fever, diarrhea and dehydration. Feces are usually soft, then watery fluid and after 2–3 days contains mucous , blood and intestinal lining.
I hope that this may shed some light on the situation. Please let me know if this is helpful.