Didn't think i'd ever ask this, but are erections normal for a 7 year old?

Hey everyone, i'm a mother of 2 daughters aged 8 and 10, and i'm babysitting my 7 year old nephew for 3 days while his parents are out of town. I was too embarrassed to call up his mom (my sister) to ask about this, so I thought i'd ask here.He sleeps nude at home and when his mum told me this, I said i'm fine with it because i'm bathing him so i'll see him nude anyway, so he's free to sleep nude so long as it's only in the guest room and not around my daughters. I went to check up on him this morning and he was awake, playing on his phone with his bed sheets off. His had a very visible erection pointing straight up and it really caught me off guard because I assumed that erections only start during puberty.Is this normal for his age or is there something bad going on here. I'm concerned because I don't like the idea of him causing it himself because he's much too young to be having sexual thoughts or feelings. Can anyone please tell me if this is natural or suggest any action I should take here. Thanks via /r/Parenting http://ift.tt/2xx7aRx