The man, in this case, should visit his doctor. There may be a medical problem causing the inability to complete - e.g. he has diabetes. One of the side effects of diabetes can be partial or complete erectile dysfunction.
Still Wondering… has your husband been to see a doctor recently? Has this been a prolonged experience? A general physical is all that is needed, typically, to determine a medical factor for this problem. He doesn’t need to go in to see the doctor with this specific symptom. A doctor told me ED can be an early sign of clogged arteries!
If this continues, then you will know quite well that it will not be possible to complete the act. When that is the case, it is not proper to begin the act.
Please state your reference for this conclusion. From all I have read, this is not true. Short of orgasm, no physical intimacy limitations are prescribed by the church. Orgasm is restricted to a completable sex act.
As Kage-ar points out , correctly, there is a point at which a couple
should not continue if the intent (or ability) is not to complete the act properly. This “line”, is not specifically spelled out by the church and should be left to couple to discern. There are implications of this “line” as described in official church approved texts. Reading suggestions include Popcak’s “Holy Sex!” and West’s “The Good News About Sex and Marriage.”
Still Wondering… what is secondary here is “how far is too far?” What is primary here is the health of your husband. Please insist that he see a doctor about his health. If a medical reason is ruled out, please have him check out any mental symptoms. Most of sex is mental, BTW. This may be relatively new or temporary, but if it is “typical” something may need to be done. You don’t mention your husband’s age nor any other reasons this might be a problem so I’ll assume he’s middle aged and this wasn’t always a problem.
And next, what does you husband think about this? Apparently he still desires to please his wife. Is there anything he feels guilty about? Does he feel unworthy? Is he frightened of something? Many things go through a guys mind concerning the act or affecting the act and feelings have a way of building up in guys. Additionally, change in method, time or environment may also change the outcome.
Now, if you already know the answer to why and it’s unlikely to improve without medical intervention, then a read of either of the two books suggested will assist you in finding your “moral limit.” “Holy Sex!” as I understand, goes into more detail for this aspect than does West’s book.
Peace be with you and your husband.