Child maintenance

If my sons dad works part time and goes to uni part time does that mean I won’t get child maintenance if I go online to do it? I can only see information saying that if they’re in full time then they don’t have to pay but doesn’t say anything about part time so assuming that means part time doesn’t count? My bd gives me £150 a month which obviously is better than nothing and for awhile he’s been saying he just cannot afford anymore than that which I’ve believed and given him the benefit of the doubt but I’ve recently found out he’s been going on lavish holidays and been hiding it from me so I think it’s quite clear he’s making alot more than he’s letting on. He said to me that I won’t get anymore money if I apply for maintenance and even said I would probably get less as he’s in uni ect. I’m just abit worried that I’ll actually get less because what he gives me does help a lot. I don’t want to be greedy and try get more but then I’m in a much worse condition. However I feel he’s been extremely sneaky just to not give me a bit extra money that my son and I are owed. Idk what the correct thing to do in this situation. I want to be reasonable but also had enough of the lies and taking my kindness for weakness. Should I apply for child maintenance or shall I just be happy with what I’ve got?