Blood in urine is still normal even at 5.5 weeks after surgery. It is still considered normal up to 8 weeks after surgery.
An orgasm simply doesn't put all that mush stress on your plumbing. The no sex caution is more due to the physical exertion of sex and less than erections and orgasm. Most guys (whether or not they ever tell their wives) JO soon after the catheter comes out just to see if it still works or feels good.
Since it happens later in the day, it sounds to me as if he is still overdoing it a bit and there are some bleeding capillaries somewhere. Everyone heals at a different pace and it may be that your husband is just too active and perhaps got back into life a bit soon.
Either way, if your doctor says don't worry about, don't worry about it. We tend to panic at the sight of blood coming out of our bodies, and rightfully so. But in this case, listen to the doc, put his feet up and rest.
I have to admit, like all men, I did the same thing. Now that it is all over, I have to ask myself, why is it that I DON'T listen to the doc when he says not to worry about blood, and I DON'T listen to him when he says to rest and heal. Hmmmm... a male trait to not listen?