Krdytkyu

I am making a new player character and in their backstory, their brother draws the Donjon card from the deck of many things:

Donjon. You disappear and become entombed in a state of suspended animation in an extradimensional sphere. Everything you were wearing and carrying stays behind in the space you occupied when you disappeared. You remain imprisoned until you are found and removed from the sphere. You can't be located by any divination magic, but a wish spell can reveal the location of your prison. You draw no more cards.

The card specifically says they leave all of their items behind.

How long someone could survive in the sphere? Since it's an enclosed sphere and they have no items, couldn't they starve?

I want to hint that he is still alive, but since it's been two years in-game since he drew the card, I don't know whether he would be alive or not.

Indefinitely

He is almost certainly still alive:

You disappear and become entombed in a state of suspended animation in
an extradimensional sphere.

Emphasis mine.

Suspended animation is usually understood to mean that you are unaffected by (and unaware of) the passage of time - neither starving nor aging (and thus not dying of old age). From the Wikipedia link above:

Suspended animation has been understood as the slowing or stopping of
life processes by exogenous or endogenous means without terminating
life itself.

I should point out that technically, suspended animation can be as limited as hibernation, in which case body processes are slowed, but not stopped, and so given enough time, the person may age/starve and die.

However, it is more likely that the intent is for that of the usual SciFi/Magic interpretation of suspended animation where time does not pass. This is indicated by the Sequester spell, which clarifies "suspended animation" a little:

If the target is a creature, it falls into a state of suspended
animation. Time ceases to flow for it, and it doesn't grow older.

Again, emphasis mine.