Child abuse is linked to unrealistic expectations of a child's understanding and development.
I agree that this is something that needs to be discussed as a matter of urgency, but as calmly as possible, given your natural shock at what has happened.
This is one incident that is definitely a disproportionate and innapropriate reaction to what is normal behaviour for a child of 18 months. Have there been any others?
Much depends on how your dh feels about what he did. Does he realise how wrong it was, or does he feel his response is justified?
If he just lost it because of tiredness, stress etc and now feels absolutely gutted and mortified, then you have a starting point to deal with the situation and develop some coping strategies.
If, however, he thinks his behaviour is reasonable/acceptable, then you have a much bigger problem and need to get some professional advice. This kind of behaviour can escalate, and usually gets worse as the child gets older.
YANBU, but need to work out where you go from here.