Surely once the divorce is through & the access terms are set, he'll be able to relax? She could still cause difficulties even then, but the law would be on his side...
One male friend of mine was cheated on multiple times by his wife & when he finally filed for divorce she was so furious that she managed to get the house, the children, the car, the dog & £100,000 a year & he gets his children one weekend a month.
Another male friend who was walked out on by his model wife as he couldn't afford boarding school for all 3 children and 2 4x4s and 3 holidays a year. Her parents told her she could get better & to screw him for everything, so she did. Despite never having taken much interest in the children previously... They now spend a lot of time with nannies while she 'networks'.
Equally, I know women who've had really disadvantageous settlements from arsehole ex husbands.
My point is: your partner's fears may be justified, and it may be a question of placating his wife until his rights are set. It may be less lack of positive commitment to you, and more fear of what his wife may be capable of.
Also I think he's right that his boys may resent him being with you & thus be difficult, all the more so if his wife is being poisonous about him. But it's up to him to make sure that his boys behave in your house
Finally, some women do find the fact of their children spending time with another woman very painful, even if they were up for the divorce. She will get over it eventually...