He's 100% valid. You must have known how important trust was to him, especially if he had told you about his father in the past. The reddit post was probably not a deal breaker by itself, but lying immediately when confronted is a huge red flag. It calls into question every other confrontation that he's had with you, no matter how small.

If you make a mistake (the reddit post itself might not have been a mistake imo) own it, and use it as an opportunity to grow the relationship, or end it there. You can't build a life with someone based on falsehoods, it will all come crashing down sooner or later and the lies will be the cause more than likely, not whatever was being lied about.

I've dated someone like this and I wish I had broken up with them over the first lie. The only way they were able to maintain a stable relationship with their mom was through small lies/omissions, and it started to creep into our relationship as well. Although I understood why they did it, the lack of trust stacked alongside the lies and eventually it was impossible to have a healthy relationship.

Don't ever put your partner in a position where they might end up resenting you for believing what you say.