Well, although I don't have any exact numbers on sperm's calories (they would differ extremly anyway) but ...
... I can assure you that sperm is actually rather healthy.
The taste and exact composing depends on the man's lifestyle (alcohol, smoking, etc.) and nutrition of course but in general ejaculat consists of many hormons and proteins your body needs and uses. Those components also (may) have an anti-depressant effect on women.
So don't worry about any possible fattening effect. Have sex after (and thus burning the calories) and enjoy his boner!
