Here are a few more body language clues to help
Eye contact (already mentioned I know), this will be direct from a distance as well as when closer & talking to you. If he is shy, then he may look away briefly. Boys do not usually do eye contact so it is a easy & good indicator. You may also notice that the pupils of eyes dilate too ( that is a very good sign as he really likes you, but make sure that its not because its dark!)
Eye brow flash, takes experience to spot this, but eyebrows lift up momentarily to those who we like. I must admit not to have seen this!
Allow you within his personal body space, you can test this by subtly moving back a little & he will most like move forward.
Grooming, he will comb/smooth hair, tuck in his shirt etc in your presence. Also he will straighten up & face towards you.
He will also start to copy/mirror your body language.
It might be a good idea to watch how over couples interact and you should see many of these in action!
I do hope this helps, but one piece of advice, never attempt to fake your own body language, it will nearly always be felt as strange by the recipient.
Thx that was good advice