Can I let my 9months old eat cakes (non cream ones) when I have not let him try eating hard boiled eggs yet? My concern is, cakes contain eggs. Or is it advisable for me to feed him eggs first?

Having cake that contains egg is safer than directly eating a boiled egg actually, since the amount and concentration of egg is low & baking egg makes it less likely to cause allergic reaction. I'd be more worried about the sugar content but small amount should be fine !