ginnedupmummy - your 1st post did shout pretty loudly that you'd been hurt in the past, so I'm not surprised that you have been wary. I commend the fact that you put your chid's interests first. But sometimes it is better to hear an opinion from someone who doesn't have a vested interest in the situation, not someone who knows you and will tend to take a softly softly approach so as not to hurt your feelings.
Talk to your dp - then sit down and talk to the children and tell them that he wants to help you care for them and that includes correcting bad behaviour.
As your dp has lived with you for 2 years hopefully you feel you can trust him. Not all men are bastards, are they?
It is very important for your boys to have a good male role model - if you are forever undermining him then you are sowing the seeds for a lot of trouble when the testosterone starts to kick in - that's if dp hadn't left before then as he didn't feel that you trusted him.
He clearly wishes to be a Father figure to your children - LET HIM if you trust him - if not you shouldn't be with him. By all means if you don't like him shouting at the children then ask him to tell them off and send them to sit on the 'naughty stair' or whatever you believe - but that's a different issue.