The 18-400 zoom is a digital DX-style lens, so lets assume an "EOS 3" is some form of a Digital Rebel of the EOS T3 variety.

I'm not going to Oshkosh this year, but went for two years running for the entire 7-day week before last year's cancellation.

A few general tips for easy(er) ways to get great airshow results are:

1. Shoot in Shutter Priority with the EC set to +0.7. Put the camera in AUTO-ISO. Use Evaluative Metering.

2. For prop planes, adjust the speed between 1/200sec and 1/320sec. Use the slower speeds for slower planes like T6 Texans (WWII) and the faster speed for high-performance planes like the Extra 300L. For all jets, adjust to 1/1000sec initially. When the modern USAF fighters do their high-speed passes, bump up to 1/1600 sec. The hardest thing to remember is to adjust your shutterspeed every time the plane types change back n forth. The example below is 1/320sec, one could just default this speed for all prop planes. Use 1/100sec for helicopters.

3. Set the AF drive to AI Servo.

4. Set the burst mode to High-Speed Continuous.

Track the flying plane with the shutter half-pressed to engage the AI Servo and press the shutter full as the plane reaches the closest point of approach, let the camera rip 3 to 5 frames at a time (1 or 2 seconds) as you smoothly pan with the plane as it passes through the closest point. Skip the 'from behind' shots.

For position, the earlier you get to your desired spot the better. People will start 'holding' spots on the flight line as early as the gates open at 9am using empty lawn chairs. I don't shoot from a chair, so I've been to the flight line to physically hold my spot for as early as 12pm for the 2pm show start. The 12pm 'hold' is to get a spot more toward the center, there is always open space at either far-ends of the runway where all the flying occurs.

The grounds have tons of planes for ground shots, so many it's overwhelming. The Warbirds area is amazing. If someone was there for just one day, get there when the gates open and tour through all the warbirds (WWII) and then plan to get a good place for the airshow. Have plenty of back-up batteries and cards.

Here's a fresh image from Oshkosh in July 2019 at 420mm on a full-frame camera. Your son's 400mm lens on a Rebel can capture the same framing at 1/320 sec if positioned on the flightline.

Yak-110 by Paul Sager, on Flickr

I checked, simply by typing in Tamron 18--400 lens on a search engine and got this:


Home
Lenses
Digital SLR Lenses
Canon EF Mount Lenses
Currents: Tamron 18-400mm f/3.5-6.3 Di II VC HLD Lens Canon EF

Tamron 18-400mm f/3.5-6.3 Di II VC HLD Lens Canon EF


The lens is on offer from Park Cameras here in the UK and is listed as an EF fitment, not EFS.
I think that might suggest there is a Tamron 18 -- 400 lens that is suitable for full frame Canon cameras.

I checked, simply by typing in Tamron 18--400 lens on a search engine and got this:


Home
Lenses
Digital SLR Lenses
Canon EF Mount Lenses
Currents: Tamron 18-400mm f/3.5-6.3 Di II VC HLD Lens Canon EF

Tamron 18-400mm f/3.5-6.3 Di II VC HLD Lens Canon EF


The lens is on offer from Park Cameras here in the UK and is listed as an EF fitment, not EFS.
I think that might suggest there is a Tamron 18 -- 400 lens that is suitable for full frame Canon cameras.