Mmr question

hi all, wonder if you could help.
my son has his mmr tommorow slightly later than usual as he had a chest infection when it was originally due so cad to cancel.
the only thing i am worried about and i keep getting conflicting answers from drs and others in the medical and herbal profession is that he has already had the measles.
he had measles when he was 10 months old (awful!) and suffered bad from them. he also suffers terrible from chest infections he has had 7 since he was born and gets quite poorly on them.
i just don't know what to do, some say give him it as he already has some immunity against it and others say don't as it could make him quite poorly.
i know i could get them seperately but they arent licensed and i didnt want to have to give him 2 chances to suffer from needles with high temps etc
has any one else had a similar thing???

rach