My son doesn't want to sit!

Barbara
So my son is 13 months and he recently started to let himself fall back every time I sit him down. It's like he doesn't want to sit anymore. I'm not sure if he does this on purpose or he is losing the ability to sit down. Anyone else going through this? 
He's behind on his motor skills. He won't crawl or get in the seated position on his own... but he was sitting without assistance. He also doesn't seem interested in standing or walking and he hates tummy time. I'm thinking of making an appointment with his pediatrician, hopefully he's just going through a phase or is just... lazy. I can't help it I'm really worried. 

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