Would I use "Aishita" or something else? The child is a 16 year old.
Unlike 恋 (koi) which refers to romantic love, technically speaking, you can use 愛 (ai) to refer to familial love. But it's basically a fairly big word, and I believe most 16-year-old boys would feel embarrassed to use it. 大好き (daisuki) should be much safer most of the time. 愛 might be safely used if the writer is mature and the song contains stiff words or words related Christianity/philosophy, but your song does not seem to be like this.