That time between about 7 and 12 months is when separation anxiety starts to happen. It's likely not so much that he forgot to self soothe, but he is just desperate to be close to you. Around that age, they start to be able to cognitively understand that you are separate from them. Up until then, babies are likely to think you are literally the same person. They don't really have the brain development to think any differently. But once they develop the concept that you are separate from them, when you go away, you're literally gone forever. They can't remember you (which is called object permanence, usually that develops around 11+ months), which is why they have separation anxiety. It's just a really tough developmental phase when they really struggle when you're apart, usually paired along with other developmental stuff, like crawling, standing, walking that disrupts sleep. So their brains are so active, they can't settle and sleep, and get really distressed without you there because they can't really understand that you won't just not come back. So just do whatever works to help settle him and make him comfortable and secure for now. It will get easier again when this phase passes.
