Hi Kev,
Yes, I am still around. I guess 'lurking' is the netspeak term!
Bottom line. Just clear your son and leave out the doubts and fears.
My first answer would be to, scan your son, and the energy is at it is. If there is a physical blockage, you will detect it. Within that, perhaps what did you feel, or what came to mind, 'if' anything does/did. Maybe some insight...

We know that many blockages come from emotional source. Knowing that, doesn't mean you switch to academic mode and work with information, as opposed to dectecting(feeling the energy) and clearing. Don't switch to "thinking and clearing"!
We also know that we can help someone heal, but they have to heal themselves.
If there is an emotional source to this blockage, it is your son's transformation and understanding that will clear the source cause. I wouldn't add that 'fear' factor into the mix. It will get you running in circles, thinking too much.

Some five element charts speak of positive attributes in regards to the five organs, along with the negative ones. One positive emotional attribute associated with lung energy is courage. Maybe self confidence , or being self conscious about something relates to that. Also, your behavior or mannerisms around your son, while doing the clearing , should be free of any judgements about any possibilities that make you feel frantic in any way, or nervous, or doubtful, as you clear his energy.

It is absolute, that you 'can' clear his energy whether you can detect the blockage or not. Just go through the steps as given.
It would serve you very well, to focus on this detecting energy thing. Put your energy there, instead of looking elsewhere for clues. The energy tells you a lot. Try working more with long distance healing regarding your son. Get in a quiet place, relax, and take you time scanning him and doing the clearing. Make it a nice meditative experience that helps both you develop as a healer, and also cover keeping your son's energy clear. Understanding comes with time, and practice. Whether it is your own, or in regards to someone else. Practice the practice. Don't leave out the detecting part. It is important for you to work with that and feel comfortable about it. Your intuition can grow as a result of your practice, and that can come into the detecting and scanning aspect, as well as what energy you put back in, whether in a session through your intention/visualizations/prayers, or as understanding given to help someone transform.
love,
gallen