Using two classes at the same time (using a "mech")

[Devilizh]

I had a crazy idea for a character and my DM and I are having trouble balancing that idea, so I need help.

The idea is to have a human boy character with a homebrew class of Inventor, but him having a "mech" that changes his stats to the ones of a Barbarian when using it. It is going to be like multiclassing but the character won't have access to both classes' features at the same time.

the idea is to have two different set of stats for dexterity, constitution and strength

We were thinking on having both classes level up at the same time but having a limiter on using the battle armor in combat, but that seems weird. We also thought of leveling the inventor class every two levels in the barbarian class.

Any ideas on how to balance it, or at least make it playable?

[Kane0]

So along the lines of an Ironman concept? There's plenty of ways you can do that.

The simplest is to be an inventor and when using the mech have a seperate statblock on hand that you use, much like a Druid's wildshape. As you level up and your mech gets better this statblock improves. You can use Druid Wilshape and the DMG monster building rules to approximate what that statblock should look like.

[DarkKnightJin]

Sounds to me you're trying to combine the Wildshape from Druid with the scaling animal companion from Revised Ranger.

Using a statblock for the armored up version sounds about right.
But, the boy would have to choose which set of skills he wants to use, outside of a few 'passives' that are designed to work in either 'form'.