I'm not sure if you've responded to your ex because I've been late in replying to your question.
His reasons might be genuine. Many men don't want to commit in relationship or marriage when almost everything doesn't seem to go well in their lives. They feel they don't want to be able to take care of their partners and their future children.
Nonetheless, that is not justifiable reason because he should have involved you and the two of you would have known what to do e.g. going slow in the relationship.
On the other hand, it might not be genuine. Maybe he didn't want to be in the relationship.
The only way you can know if his reasons are genuine is by looking back at the relationship. By reflecting on the relationship, you will know if there was something that led to the breakup. Was there a change in his behaviour or attitude? Was the relationship faced with war of words?
If you've not responded, you can tell him you've received his message of the breakup but you'll unable to respond to it. Therefore, he should give you time to digest the information. After one week, you can respond to him. After this period of reflection, you can contact him and tell him you've thought over his decision to end the relationship. If he can't give the relationship another try or to can go slow in the relationship, you won't force him.