I don't understand all the froth and venom towards the OP here (putting to one side for a minute her previous posts)
-she is 33 weeks pregnant and wanted to prepare meals for when the baby arrives
-she is recovering from an eating disorder so is probably very specific about these meals and (rightly) doesn't want to fall into old habits
-she works from her own place most of the week as her work takes her into the evenings and she has sensitive content to discuss not appropriate for a 3 year old
-she left the meals in DPs freezer as she has ordered a larger one for the new house they have yet to move into
-her partners child is at nursery for full days so (if anything like ours) he gets 3 full meals and requires something before bed. OP assumed this wouldn't be a full meal (differs child to child)
-OP has made separate meals for DSS before but they haven't been eaten so she stopped and assumed (GASP) that the father would prepare something
-OP has only prepared meals for 2 as they both eat when the 3 year old is in bed post 7pm (again, completely normal for many parents)
AIBU: her DP ate into these meals and fed them to his son knowing they had been prepped for when the baby arrived
Conclusion: your DP is a twat (and it sounds like this extends beyond eating the food)