Really?
Realistically speaking, no father would support his son having a relationship with Bella. It's not a question of age difference, that's the least of the problems since relationships with long age gaps exist and are increasingly common.
The real problem is that MC has gotten into a relationship with a married woman... even worse is that MC has gotten into a relationship with a woman who doesn't even know if she's married or a widow... what if one day her husband comes back?
MC clearly hasn't thought about this possibility or is avoiding thinking about it... it's like Bella... she's moving forward with her relationship with MC without thinking about tomorrow... a recipe for disaster.
If Neil doesn't point out these factors, Neil will be a bigger idiot than the MC.
Well seeing as Neil and Lynette didn't get married, I wouldn't expect him to stick to the marital status for the sake of sticking to it. If James comes back Bella can just divorce him. What Neil will focus on is the emotional side. Is Bella following her heart?
And that is exactly what he would worry about with all the girls. Is the sheltered rich girl really in love with his son or is he just a convenient stand-in for her naive fantasies? Is the party-girl taking his son seriously or she just having fun? Do the bisexual girls really care about is son as much as they care about each other?
MC has thought about Bella's husband at quite a few points, but refrains from pushing Bella on it because that's the advice Jill gave him. MC and Bella actually discuss this after the DIK TG BBQ albeit briefly. It is not a topic that has been ignored. And quite frankly Bella is one of the few characters who does worry about the future. That discussion about the "elephants".
Honestly I have to wonder if you have paid proper attention to the story. When was the last time you actually read the dialogue instead of Ctrl skipping to the scenes?