If you are having him hold that B12 in his mouth? Sores can result. The oral can be swallowed, instead. Some people get very poor gum and mouth health, from long term illness and nutrient deficiencies.
Vit C, zinc and Co-Q10 are often used to improve oral mucosa.

The injections...are they cyanocobalamin? Daily use would expose the patient over time to cyanide, as it is cleaved off to activate the cyanocobalamin. Some people cannot handle this much cyanide (which is a poison) metabolically.

I don't think daily shots are needed past 7 days, as long as the oral is being done, and correctly on an empty stomach.

Also rapid treatment with B12 may lower potassium levels, temporarily, as the B12 stimulates the bone marrow to make more red blood cells.
Low potassium can lead to leg cramps and generalized weakness. So eating a diet rich in potassium is important.

What other drugs is he taking? That could factor into response and side effects too.