Assalamu alaikum, depending on how practising your son is, you and he might want to seek advice from his local mosque. Different imams and scholars might feel differently.
Personally, I'd have concerns that a purely Catholic ceremony would potentially miss out the key bits of the nikah, and could involve the Muslim spouse having to say things and make promises within a different faith, which could be problematic! I'd also be concerned that if Islam is not going to be included in the marriage ceremony, what precedent does that set for its inclusion in life after the ceremony? It's a bit different from having a purely civil ceremony (with a separate nikah), to have a ceremony entirely within one person's religion but not including the other person's...
When my DH and I were planning our nikah (both Muslim but my family is from a Christian background), the imam at our mosque would have been happy to be involved in a cross-faith ceremony to include a prayer or blessing from my parents' church - we didn't include it as neither of us are Christian and we wanted our nikah to represent us, but it was nice to have the option suggested to us.
If the bride and groom are from different faiths, maybe a ceremony could reflect both? Maybe something in a civil setting like a hotel with someone from a church and an imam both involved in the marriage ceremony?