Although some men do use this as a "line," I do think that your boyfriend is quite possibly telling you the truth.
Many men do find it difficult to maintain an erection when using condoms. Sometimes this is because of anxiety, other
times it is due to loss of sensation. However, most men can also overcome this problem. I do have a couple suggestions that
you can try though:
1) Help him put the condom on. Many men find themselves getting anxious about putting the condom on correctly and
lose their erection at that point. So, tell him that you will take care of it. Stimulate him any way that he likes and
then, in the process, slip the condom (which you have opened and ready) on. If you want to get really advanced, you can
even try putting it on using your mouth -- just be careful with your teeth!
2) Put a dab of lubricant in the inside tip of the condom before putting it on him. This can enhance sensation
tremendously for the man. Just be careful to avoid getting any on the upper rim or the condom might slip off during sex.
3) Surprise him with a box of novelty condoms. There are many types of condoms that are downright amusing.
Glow-in-the-dark, flavored, shaped... treat them as a sex toy! Although some of these are not designed for safer sex,
it can help overcome his anxiety.
4) Purchase an erection ring. These bands go around the base of the penis to increase hardness and help maintain
an erection. An extra benefit is that they can help keep a condom on if the man becomes partially flaccid.
Most of all, continue doing what you have been doing -- being supportive and talking to him. Assure him that you understand
but that you also want to help him overcome this problem. Remember, condoms can be fun and they do a great job with protecting
us against sexually transmitted infections when they are used consistently. Good luck to both of you.