Hi C.,
Children that old LOVE stickers AND paint, so give him a jubo paint brush and let him dab away with the brush on watercolor paper. Just use one color, so clean-up is easy. Then let it dry or blow dry it with your hair dyer. Once dries, let him stick foam stickers, regular stickers, etc.
My daughter made a beautiful painting and sticker poster, I actually framed it and it now hangs in the downstairs bathroom!
I always use watercolor paper as it is much more absorbant, and doesn't wrinkle up.
Another idea is trace his hand and feet and cut out the shapes. You can get a twig, stick it on paper, and then color the hands and feet and stick them on like a tree. You can also use string or yarn to make a nest and make a cut out bird. This teaches them about nature, so when you are walking, you can tell him, that is like the tree we made at home. See the leaves and bird!!!
Just an idea! Hope you have fun. 18 months, they are absorbing everything!!!