這是一個千載南逢的好機會，應該好好加以利用

The most import thing for this incident is not how your son should react, it's what he think about it. If he got the correct thinking (from the business perspective), how to react is out of question.

Someone suggest "let it go". Although I agree in general you should let your son learn his lessons through his own expereince, in this case I disagree. As you mentioned in your carrer there were multiple cases someone took credits from you and you choose to accept it. But those left bad taste in your mouth. And appearantly you can't forgot about it. Incidents like this not only make you mad about the people took credit from you, but also make you not forgive youself because the way you choose to deal with it. Believe me, this wont' go away unless you learn the right way to deal with it. It's the time for your son to learn it. Choosing doing nothing isn't an option.

Someone suggested "nobody can really grow if his/her ideas and feelings are always "corrected" by someone.". This is totally true. But the thing is you maybe the person always corrected your son's feeling in his life and he already got used to his feeling is not cared. Now someone else not care about his feeling and he is ok with it. You need understand the true meaning of "who cares?". Is that he didn't feel offended at all? or he felt offended but choose to hide from reality it like you did? or is this something he already had a plan in his mind so he don't want to talk about it with you at the moment you asked?

I suggest you make it a bussiness case. You don't offer your proposal, but you demand solution from your son - the team lead - to deal with a business situaion. 

If there is a problem in a bussiness team which impacts the productivity or integerty of the team, the problem need immediately addressed. You raise this problem to your son. You talk to your son and ask him forget about you are his Mom for a moment. You tell him you have a problem need the ream lead attention. You are a stack holder of the the project. You invested your $, time for it for a purpose not without condtion. And someone else not fairly use the team blog. The team blog supposed to use to attract public attention and to motivate the team members. However a distorted story is told and that hurt your feeling and further impacted your dedication to this project. You want him to slove your problem. Your son will learn how to deal with people, starting by how to deal with his parents fairly.