How do I keep my son entertained if hes not mobile?

My son is 9 months old and does not crawl what so ever. He doesnt walk, scoot, crawl nothing and he really has no interest in trying to be mobile. Although I can tell he gets frustrated when he cant do something kids around him are doing or cant get to something. Its actually making him very fussy 24/7 I think cause hes frustrated. What are some things I can do with him to still work on his development but be fun and maybe enourage crawling more? Has to be something thatvhe can do standing or sitting unless its tummy time cause thats all he can do now... Please help! I always feel bad just sitting him on yhe floor with toys aroubd him I dont feelvas if its helping his mind and devlopment at all....what can I do!?