Hi Joyce, I've got some info about feeding and when to start at my blog. You may wanna take a look.
If you're starting when your baby is 6 months, it's ok to still use the 4month+ cereal.
If he's eating well, you can slowly increase the amt given for the meal (usually you start with only a couple of teaspoons then slowly increase). You dont need to increase the number of times he's fed semisolids so fast. Usually they need some time to get used to eating semisolids.
For introduction of new food you just need to pay attention to the 4-day wait rule.
Anyway, if your son is gaining wt well on breastmilk now, there's no need to start on semisolids till he's 6 months old.