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You can land anywhere in the EU or the Schengen area with a valid passport. There is no need for the passport to be valid beyond the end of your stay.
This is specified in directive 2004/38/EC, article 5:
Member States shall grant Union citizens leave to enter their territory with a valid identity card or passport
I can also speak from firsthand experience traveling around the Schengen area last August with an EU citizen whose passport expired in September.
Article 6 allows countries to require you to have a valid passport or national ID card for the duration of your stay, but the details vary from one country to the next. In practice, it's unlikely to be a huge problem if your son's passport expires while you're in another EU country, but it might be simpler to renew it before you travel.