Mother in law is trying to dictate what the Groom should wear

Your post has a lot of what you envision and what you prefer. What does the groom want? Just like your dress was up to you, your groom's attire should be up to him. If he prefers to wear a suit, then he should wear a suit. I've been to many, many weddings where the groom wore a suit next to an elaborate ballgown wedding dress, and it looked just fine! If he prefers to wear a tux, then he should make it known to his mom that he prefers a tux. There's an old saying when it comes to difficult conversations - "blood handles blood". By letting him handle the conversation with his mother, it will hopefully prevent any hard feelings/animosity between you and your FMIL. Your groom should also choose who he wishes to go shopping with. I think a lot of men prefer to go as a group with the groomsmen, but some may want that bonding experience of shopping with a parent (like many brides do with their moms). I would just let him know that you are fine with whatever he wants to wear and whoever he wants to go shopping with, and let him handle it.

The groom's attire is the business of the groom, not his mother, but other than that FMIL is the one who happens to have the etiquette right. You dress for the formality of an affair, not to match the bride, for whom the dress stands on its own. If an affair is not formal, for example, black tie, then common as this is to see no one properly wears formalwear including the groom, groomsmen, and the dads. For that matter, the fathers are not part of the wedding party and no one should be dictating what they wear, either.

That makes sense I guess. Part of the issue that I've been having with him though is that he truly just does not care. In every step of the wedding planning process I try to get him involved and ask for his input. I know that it's his wedding too and I really want and need his help. But every single time I ask for his opinion he just doesn't really seem to care much. His whole attitude is just "I don't care, whatever you want is fine," which doesn't help me at all in this scenario. So far I've been able to manage but this is one thing where I need his opinion and he just doesn't care.

That's true. It's just hard because our parents are 100% (my dad and his dad) paying for this wedding and it's tough to know how to involve them and to what extent.

I think you should be honest with your opinion. but leave it up to your groom to decide, I mean, maybe you can dictate color!? at least! lol. I plan on buying my husbands Tuxedo. I like to control things as well so i can see how this would upset you. Just be honest then let it play out.

Ugh. Grooms can be so frustrating! lol I think they've been raised with the whole "it's the bride's day" and "happy wife, happy life" mentality, so they think they are doing us a favor by letting us plan everything. I would have a sit down with him and say "up until this point I have made all the decisions. YOU ARE MAKING THIS ONE. You have until X date to decide 1. whether you prefer to wear a tux or suit (and maybe show him some pictures of real weddings with both options so he can have a clearer vision of what they both look like next to a dress) And 2. who you want to go shopping with for attire."

It may be helpful for you and him, or him and his best man (or even just him alone) to a store that carries both options. Then he could try on a couple suits and tuxes and see which he prefers, without the pressure of anyone else's opinions trying to sway him. Then he could join the rest of the guys for the "official shopping trip" after he knows which option he's wanting.

Also, you don't need to worry about the guest attire formality when dressing the groom. It's actually quite common and acceptable (and supported by wedding/etiquette specialists) for the groom to be dressed in a more elevated fashion than guests. So you don't need to be hosting a black tie wedding for the groom to wear a tux. However, it is generally accepted that you would be hosting an event that is just one level below the groom's attire (so one where your guests will be dressing in formal or cocktail attire).

Our guests will most likely be showing up in formal attire so that what kinda what I was thinking of when I suggested a tux. I think I'll just force him to actually make the decision on this one lol.

I know it's common these days and that most people wouldn't bat an eye but, but strictly speaking, it is not traditionally appropriate etiquette.

OP, Is your wedding black tie or black tie optional? If so, then yes, he should wear a tuxedo. If not, why do you think guests will show up in formal attire? That's unusual.