For a groom bridal shower outfit: she is seated. He is not.

A groom bridal shower outfit is usually being chosen for the wrong event, because if he is there for the afternoon it is a couples shower rather than a bridal shower. This site is emphatic about the distinction: the name is the only information a guest has about who else will be there, and calling one the other is the most common reason somebody arrives expecting a different afternoon.

Confirm which one it is before anything else

A bridal shower he attends for the last twenty minutes to say hello is not an event he needs an outfit for. A couples shower is a full afternoon with a mixed guest list, and it is the one this page is about.

Ask the host directly. It also tells you the guest list, and the guest list decides the formality more reliably than the invitation wording does.

The bridal shower gifts are the structural difference

The bride's shower dress has to survive two hours seated, photographed from the front, with a gift opening in the middle. That is the whole test on this site's shower dress page and it is why so many product listings give the wrong answer.

At a couples shower the gifts usually do not get opened in the room. They go on a table, they are opened later, and the afternoon keeps moving. So the groom is standing, circulating, probably going outside at some point, and appearing in photographs taken from every angle rather than one.

Which makes it a standing test

That points at different clothes than the seated version does. Nothing that only works when still, nothing that needs adjusting when he stands up, and nothing so structured that an afternoon of moving around leaves it creased across the middle.

It also means shoes matter more than they would at a seated event. Three hours on a lawn or a patio is the situation in which the wrong shoes become the only thing he thinks about.

The bridal shower room decides the formality

The venue sets the level for him exactly as it does for everybody else: a living room means a good shirt and no jacket, a restaurant means a jacket he can take off, and a hotel or a tea room is the one place a tie makes sense.

Someone dressed for a hotel in a friend's living room is overdressed by two full steps, and that is more conspicuous on the groom than on anybody else, because people are looking at him. Ask where it is, then dress for the room.

An outfit one notch above the average guest, and no further

He is not a guest and he is not the host. The calibration that works is slightly more considered than the room's average and never more formal than the people hosting it, who are usually the ones who chose the venue and dressed for it deliberately.

In practice that is a shirt with something over it rather than a suit, in almost every setting a shower actually happens in. A suit is right for a hotel room booked for the purpose and wrong everywhere else, and the failure mode of a suit at a backyard shower is that it makes every other man there feel underdressed.

Coordinate the outfit with the bride, briefly

Not matching, which reads as a costume in every photograph. One conversation about the formality level so they are within a step of each other, because a bride in a dress chosen for a tea room standing next to a groom in shorts is the photograph they will actually have.

That conversation takes a minute and it is the only coordination worth doing. Beyond that, matching colors deliberately is a decision that ages badly and that neither of them will defend later.

The groom outfit things that actually go wrong

Being too warm, almost always. Showers happen in the afternoon, often partly outdoors, often in months when it is warm, and a jacket that looked right in a mirror at eleven is carried around for three hours.

Something he can take off is the answer, which is the argument for a layer rather than a single formal piece. And new shoes, which is the other reliable failure. Neither of those is a style question and both are the ones people actually remember.