He lays on me then growls (mix)

SO and I just got a 2-year-old mutt (alsatian+akita mix), he previously belonged to SO's aunt. He's a sweet, smart, snuggly boy 90% of the time, but sometimes he does this weird mixed-signal thing.

He'll sit or lie down super close to me, often touching me, and then he'll get angry if I touch him or get closer to him. Sometimes he leaves 0 space between us and if I try to move, even though I can't move without getting closer to him, he'll growl. He'll put his head on my feet but growl if I pet him. He doesn't bare his teeth, but his hackles stand up. It's not about a specific area on his body. And right after he usually looks at me sheepishly, wags his tail, shows his belly or gets closer, as if he wants affection now... then growl if I give him any.

Again, most of the time he's fine and we have no clue what triggers this.

SO and I have been around dogs our entire lives, neither of us experienced this before. We Google and speculate, but we honestly have no idea how to deal with this.