1
He is getting stronger every day.
To sign this, you would point to the person, sign 'GET', and then use a forceful version of 'STRONGER'. You can emphasize the increase in their physical power by making the sign slightly larger or more intense to show that the strength is building over time.
2
You need to be stronger to lift that box.
Start by pointing to the person for 'YOU', sign 'NEED', and then sign 'STRONGER'. Follow this by mimicking the action of lifting a heavy box. The sign for 'stronger' here sets up the physical requirement needed for the action of lifting.
3
My coffee is stronger than yours.
Sign 'MY COFFEE', then 'STRONGER', and point to the other person's coffee. In ASL, you can show the comparison by signing 'STRONGER' with a definitive, sharp movement, making it clear that your drink packs more of a punch than theirs.