If he wanted to see his sons he would have.
If she's genuinely ill and they are worried about the dcs catching something from her, I could kind of understand why he'd not want to take them to her house, but that doesn't mean he can't see them at all, he can take them out somewhere, it doesn't even need to be anywhere expensive. Girlfriend is an adult and doesn't need him to look after her, the contact isn't for the dc to see her, it's for them to see their Dad and her being ill isn't an acceptable reason to not have contact. What does he thinks happens when you are ill?
My Dad would do this all the time too, it fucked my relationship with him up and as an adukt I almost went NC, he died before that happened. My Mum used to be angry but wouldn't show it as he'd say she was poisoning us against him, it wasn't about her being annoyed that she had to have us that weekend, it was because she had to watch me break my heart when he didn't show, she had to wipe away my tears and build me back up and that next contact time I'd be excited and he'd be the best dad ever, and then she'd know it would happen again and that she would have to clear his mess up again.
She says she wished she stopped contact because she'd hate seeing me think it was me, that something was wrong with me, but I know if she'd done that then dad would say she's a bitch and tell everyone what a cunt she was and he had this way of everyone falling for his lies.
Have a great day with your DC 