Hello worriedmum
First of all, it's fantastic to see a mother that is taking this problem seriously. As you may have read on this forum, very often gynaecomastia is a hidden condition that many men live with and never tell anyone in fear of rejection or dismissal from family and friends. Your son is a lucky young man.
Conversely, gynaecomastia is not the end of the world, far from it. Breast tissue in men is very, very common. Around 60% of all adult men have some degree of breast tissue. For most, it's not a problem visibly. For others it can be visible but is still not a problem. For others, even the slightest breast shape or protruding nipple can have a devastating impact. It's all relative. From my own experience, the more educated everyone is about it and more 'normalised' the issues surround it are, the less psychological impact it will have on not only the man that has it, but those around him!
So what questions should you be asking? Your son is currently going through some blood tests. What they will be looking for is a number of hormones and check there levels to see if they are in the normal range. The main hormone they will be looking at is testosterone to see if it is within the normal range. As you will know, hormones can be very erratic during puberty which is why you have been told that things may normalise as your son comes through into full adulthood, which may not be until he is around the age of 21
You say that the paediatrician said his hormones were 'off'. What does this mean? Ask him or her which of his hormones are affected, if they are likely to normalise or if it is a chronic condition (hypogonadism - or low testosterone).
Personally I am surprised that the paediatrician would be happy to refer to a plastic surgeon. It is unusual for males to be surgically treated at such a young age. Most of the time, doctors prefer to wait until the end of puberty. But each patient is very different from the next. I don't know your son's personal circumstances or the extent of his gynaecomastia. It would be worth asking your pediatrician what the likelihood of your son's gynaecomastia going on its own accord is rather than go through the trauma of the surgical route.
If your son is indicated for surgery, do you intend to go via the NHS? If so, find out who that surgeon is. Will it be a general surgeon, a breast surgeon or a plastic surgeon? Does this surgeon have significant experience in sculpting male chests? Male breast surgery is very different from female breast surgery.
Once you have progressed to the next stage - ie. surgery is definitely indicated and you are due to meet with the surgeon, it might be an idea to come back to this forum and ask "What should I be asking the surgeon?"
You probably still have a load of questions or feel you have a knowledge gap. Feel free to post on here as much as you need. Comb through the Gynaecomastia Talk and Diaries sections of this forum and you will come across posts by other concerned parents (though those topics tend to be far and few between).
Hope that helps.