My partner is unable to ejaculate yet enjoys the ...

Your partner should be encouraged to see a urologist to determine if there are any physical reasons for his failure to ejaculate. Very often prostate problems interfere and age is also a factor. If there is nothing physical then he should consult a therapist.

I agree with Rock on this one. But I have to ask, is there a reason you are so concerned with your partner not finishing? Are you trying to get pregnant? I ask because often times when a partner cannot ejaculate, they don't enjoy intimate relations at all. In your case, you're having relations despite the lack of completion on your partner's side which is quite interesting.

If your partner is interested you can try looking for his g spot.

http://www.askmen.com/love/vanessa/23_love_secrets.html 

After figuring out why he's having problems you'll be able to take care of them. If the problem is physical there are different solutions for many conditions today. If the problem is psychological or something else, a sexologist could probably help you out.

Don't forget, this is mainly your partner's problem so make sure he wants to take care of it as well. 

Try oral sex and see if this helps him be able to ejaculate.  If so, keep having oral sex and this will build his confidence and he'll probably be able to soon be able to ejaculate during intercourse.  Good luck.