How to fill up my bottomless pit of a teen?

[Yestermost]

Any ideas of other foods to fill up my always hungry 14 year old. On a normal day he eats:
Porridge and toast for breakfast
Lunch: school dinner (curry, jacket pots, sandwich etc)
After school: usually 2 eggs with wholemeal toast, about 6 biscuits, lots of fruit, cherry tomatoes,
Dinner: huge plate/plates of pasta or rice with bolognasi or stir fry.
Snack before bed of toast or cereal

He will eat and eat but is classed as underweight. Not too worried as full of energy, plays lots of sport.
Do you have any ideas of other snacks to fill him up during the day?
Ideally savoury but not too salty or fatty. Thank you!

For those that are suggesting it's too much or he has a tapeworm, that amount of eating is completely normal for a teenage boy. They eat a ridiculous amount!

Add chia seeds to the porridge?

Peanut butter on bread/toast.

Rice and beans together are very filling.

I have a 17 year old ds who stands 6 foot 2 and still growing, full beard and built like a brick shit house. I could empty the entire contents of Asda into his mouth and he would still be at the fridge saying he's hungry. These buggers can eat!

Will he eat soup? A thick soup can be really filling, particularly if it contains lentils or potatoes. I think the fact it’s hot and liquid can make it feel filling. Could be had as a starter before meal or a snack. Could add wholemeal roll too.

I'd add cheese and nuts and full fat yogurt in to add calories.

My DS has always been like that and is still underweight as an adult. When he made a definite effort to gain weight, he did it by drinking daily protein shakes with added peanut butter, along with bags of nuts, toast with peanut butter, cheese.

And Aldi are great for Brooklea full fat greek yogurt in the squat 500g tubs.

I cook a big pot of veg curry at the weekend - lots of chick peas and lentils. It's kept in the fridge and kids told to nuke a bowl if hungry between meals or after dinner.

Mine were always starving, till I suggested that they were mistaking the hunger for thirst. Solved a lot of problems in our house!! And they can still eat way more than OP's son. Lean protein and lots of olive oil, and less processed carbs works for my two (19 &17).

Bagels
Peanut butter sandwiches
Cold chicken

I have a brilliant long list on the fridge now ready for shopping day. Thanks for taking the time to give me the ideas everyone!

Do you use soup mix? It’s barley, lentils of various types etc and you soak it overnight and then add it to vegetable soups to bulk them up. I also do flapjacks with added stewed fruit, peanut butter, nuts and dried fruit which are very filling.