AlohaMolly I am so sorry this is happening.
I think your first 'job' is to get your self esteem back. If that means leaving him then do it, if that is what you want. If staying helps you to do that, then do it.
I think if you can access any assertiveness training or counselling (you may be eligible under NHS due to post natal depression etc), then get some help.
You've said a lot of negative things about yourself, called yourself a lot of unkind names, please stop doing that . It's not going to help you. You are an amazing and incredible woman. Your dp chose you and you've now produced an amazing baby out of your body.
Not everyone can do that, it is fucking amazing.
It does also tend to leave a lot of difficulties around afterwards, added weight etc. And maybe some of this is what is making you feel bad about yourself, but there may be other things like your not feeling like you can trust him. Plus your not so dear partner's thoughtless comment.
Can I play devils advocate for just a minute? Did he really say he no longer fancied you? Was this after you both stopped having sex. Or was it before?
Could any of this be his way of trying to deal with the lack of sex in your relationship? Pretending he doesn't care. You said he was very ill and you cared for him. That can change the dynamics of a sexual relationship, so can suddenly becoming parents.
Yes he has behaved very badly with the text messages before your child came and again now. But as far as you are aware he has not actually cheated? Could you have a conversation in which you attempt to make things right between you, and ask him to do the same, for your sake, for his sake and for your child? Do not stay just because of your son, but stay if you can make it work, maybe?
Could you honestly talk about the lack of sex and perhaps lack of affection (hugs, kisses, intimacy etc).
" I am fat, frumpy and basically a disgrace." is this what you say to him, or imply, do you speak down to yourself around him? I know it is hard but I think this in itself may be part of the issue because you don't like yourself at the moment. You sound like you are suffering from a bit of depression, (post natal still maybe?) and maybe this is making your relationship harder too.
Believe me I am not condoning your partners flirty texts at all, but I think there is lots going on in the relationship which is not working well as well as this women making trouble.
I am not sure that he is trustworthy to be honest but I do think you may wish to see if this relationship is worth saving before you leave.