First let me say I am by no means a rash expert. My 3 yr old has a horrible rash all over that 3 Drs can't seem to figure out. She is on a med for a month then if it isn't clear they are doing bloodwork.
My guess is that they appear to be hives. How long have they been there? Any other symptoms/Allergies? That on his elbow looks like eczema,but his back really looks like an allergic reaction to something. Hydrocortisone has never worked wonders for us. You probably need something stronger.
Keep us updated and sorry I am no help. Poor baby