in the first sentace : I have had stomach ache since morning.
do you mean you have stomach ache from the morning until now ? or the tense has finished ?
When you want to say something using the present perfect tense, e.g.:
I have had stomach ache since morning. "have" is needed because I still have stomach ache.
I have had a tooth out at the dentist's, and so I can't eat now. You don't need "have" because the tooth extraction is done and finished. (completed in the past)
The problem is that lo3ti asked about the use of "have had". You gave one example in which "have had" is required (the stomachache continues) and one in which "have had" is not required (the tooth extraction not is not continuing).I think there's nothing wrong when you use a perfect tense We are talking specifically about present perfect tense, "have had". form to talk about the cause of someone's not being able to eat now - such use, I believe, also gives the idea of recency. I don't think the idea of recency is the issue.
The problem is that lo3ti asked about the use of "have had". You gave one example in which "have had" is required (the stomachache continues) and one in which "have had" is not required (the tooth extraction not is not continuing).