Elasticated Waist - how to shrink?

[NotQuiteCockney]

DS1's school trousers are track suit bottoms. They are elasticated, but DS1 is very very long and lean. Can I just sew a bit of it together? I don't mind if it looks a bit rubbish, I'm just worried it won't work. If I do this, do I try to sew the fabric? The elastic, too? Or is there another fix?

These are trousers I bought second hand (he wears through the knees) at £2/pair, so I don't mind if the 'fix' isn't undoable, or isn't perfect.

Could you do a little tuck in the middle of the back waistband?

I normally just take a tuck in the waistband at each side. i don't bother sewing the material, it just acts a bit like a pleat. My DD is very small for her age and nothing ever seems to stay up otherwise - I love adjustable waistbands, they should be in everything