If he had hard sex or even bent his penis in a slight way during intercourse it's possibly he ruptured a small blood vessel.
You also state he's had frequent bowel movement has there been any blood in the stool? If he has increased any fiber or fruit lately that would explain the frequent bowel movements which could also cause bloating. At this point though I would say those two are not related to the blood in the penis.
If this is the first time and there has not been anymore blood coming out, I feel that some how during the sex last time he may have ruptured a small vessel.
Now it is also possible to have an infection which may or may not have other signs at this moment. Only way to know that for sure it to have a swab test from a doctor.
At this point, if he's feeling fine, ie; no fever, no pain upon urination, no lack of urination and of course no more blood spots, I wouldn't worry about it too much. If it does appear again, or any of the others above begin to happen, I would have him checked out.
Stress can put a huge strain our systems too, which could be a cause for the frequent use of the restroom and bloating. Which can give each person different results.
So at this point monitor it see how he does. Let him or hopefully he will just relax for today and not worry or stress over anything.
As far as the blood on the underwear, not much they can do with that at this point. If he needed to they could do a swab collection from his penis if it came to needing a sample.
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