Hello! I'm not a teacher here, but I'll try to answer your question: I wouldn't add an adverb such as "only" in this sentence, but I'd reprase it : "he's been doing nothing but reading since morning", IF this is the meaning you want to give to the sentence.
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A senetnce like "He has been reading since morning."(only)
If I have to insert some adverb in this sentence , then what would be the most neutral position.
Should I insert an adverb after first helping verb?
Are any special rule for this?