Bf babymum is stopping him now

I'm not sure what you're expecting in response. It's obviously a very difficult situation for him. The fact that he doesn't communicate directly with her and everything goes through the 12 year old and grandmother is not a good sign at all. He would ideally be able to communicate either with her directly or through an adult instead of a child, but the most important thing is that there needs to be certainty as to contact arrangements here. That is where court proceedings come into play and, like you, I do think he needs to seriously contemplate applying for a Child Arrangement Order. That is an order of the court that specifies how and when he will have contact, which will give him more security when it comes to seeing his children. You say he doesn't want to get nasty, but it's difficult to see how it could make things much worse when they don't talk now and she is apparently bad mouthing him in front of the children. Whether that's true or not (there are quite a few things in your post that may or may not be entirely accurate), it looks like he should be seeing his children more than he is, and it's ultimately about the relationship that his children are going to have with him. This situation needs to improve for both his and their sakes, and if he can't work through that with their mother, court is really his only option.

Your boyfriend shouldn't need any advice or encouragement to do something about seeing his children; they're his sons, why wouldn't he go to court and sort out what is best for them? Giving up fighting for them is giving up on them, full stop. Then again, he can't complain when he's complicit allowing them to be manipulated and neglected and does nothing to stop it.
Unless he's thinking he might end up with custody of them is mum is declared unfit? Do you know why she has them and not him?
Court fees are listed here:
https://www.gov.uk/government/publications/fees-in-the-civil-and-family-courts-main-fees-ex50/family-court-fees-ex50

He’s tried countless times respectfully and he doesn’t want to become disrespectful in order to get his kids. She has put her 12 year old son at the front of this by also getting him involved to lie to his dad. they are a typical Asian family who don’t like the embarssment of courts etc. hope u can understand.
Her mum has said to just give it time but he said it’s heartbreaking to him. He does everything for them , she seems to only cook and clean for them and not actually parent them by taking them out the house. She locks them in the house and doesn’t allow them to have friends or go out it’s so heartbreaking to hear. Sometimes he’ll call his son on a sunny beautiful weekend and ask if they have gone out and they would say no. They only go to school and grocery shop.
I believe she may have a mental health condition because she doesn’t work so who pays her rent and also how she acts is giving someone not mentally sane. I have friends who have kids who would never behave in this manner.
Do you think she’s still angry about the cheating that happened 8 years ago. They do not communicate so there’s no issue that this could have come from.
He doesn’t want to declare her unfit as it may start causing serious problems. I mean we don’t even know to what extent she is he doesn’t see her or speak to her. Only speaks to sons or grandma.
He works full time 6am-6pm. 5/6 days a week it would be impossible for him to have them plus he lives in a one bedroom flat. Which is quite big but yeah. I mean if she was seriously unwell he would protect his children however I feel this is pure bitterness.
I don’t want to be the bf to say my bf babymum is bitter but I’ve heard what the sons say and it’s awful. The sons also want to see their dad because he actually takes them out. Can you imagine oldest is 12 and he took them to their first fun fair a couple months ago. She doesn’t work so she has no excuse to not take them out, he will give the money.