Not a teacher, but a fellow learner. My humble opinion is the following.
Doesn't it come from your Japanese thinking similar to Korean one? Koreans also say like "Watashitachi ga gare no tonari", but I think it's a unique expression of Koreans and Japanese. I think English speakers tend to say "When we aren't with him", which is natural.
1. When we aren't
beside(with) him, he pains(paints) pictures alone near the sea.
I think "and" should be added.
2. He often paints pictures about blue ocean and blue sky with some white clouds.
I think this is perfect.
3. Everyone in my family thinks that his artworks are just like real scenery.
Other teachers can help you more.