Hi... every baby development is unique and at different pace. Please be patient with him. I suggest you put some pure aloe vera gel on his pimple-like rashes to clear it.
Not to worry. Every baby development is different. My LO only able to sit without assistance at around 10 months old.
My baby till 8months then can sit on her own. Every child pace is diff. No need stress on this
Your baby will probably learn to sit independently between the ages of 4 and 7 months.
Don’t worry about that it’s normal as every individual is different.