This is pretty sick. The guy saw him small and growing until certain age, like bruh... well, I don't recommend it, it isn't cute or romantic, just pretty much problematic. (If you are reading this comment, and find relationship between uncle X nephew / Older guy that is like family seeing the romantic pair growing up for a period of time, don't read this, save yourself) Me and my vice of not reading tags and synopses
Ojisan to Ore no Koiwazurai