Hello. I have a ex-prem ds with cerebral palsy. It is ok, different but ok, and DS is quite severely affected (he is 7 and doesn't really walk). He is happy, bright, and mainstream school, but walking is just a bit too hard. He has done worse than anyone expected when he was little, and how he presented then, but again that is fine.
The first thing I would say is that 8 months corrected is not really late to sit. Also he was born later than most ex prem children who have cp. Typically the children with cp are born before 30 weeks, but of course I do understand why that doesn't matter if you have the 1 in a xxx chance child who is the 1.
Second how has he been with tummy time? Are his other muscle patterns normal, does he reach for things, kick, splash, does he use both sides equally. There are other muscle differences when they are little. Ds sat at 10 months, commando crawled at 11 months, crawled properly at 13 months, cruised at 22 months and walked at 6!
Third take him out of the jumperoo.... If he was premature it is really bad for him. I get that it might preserve you sanity, but physios hate them for all children especially premature children, and those with cp.... It really delays muscle development/strength.
The speech is also normal. Ds spoke at about 18-19 months, sentences at about 2 and a half, now I cannot shut him up.
How is he with food/feeding, did he feed ok as a baby? How has weaning been?
Also has he been ill at all, Rev? Bronchilitis? All those things add slight further delays, or can do.
However go to your gp, get some physio input. You should have it as an ex prem child anyway, and it cannot harm. In my area all scbu babies are followed up until they are 2, and if there is an issue then the more you do early the better. They will give you ideas. From memory at that age we did a lot of stretching and reaching, and repeat, reach for bubbles, rattles, bright balls etc. Also for babies they recommend massage and swimming if your child likes water.
As to whether you should worry, I could say don't but being honest it won't help having a stranger on the Internet saying don't worry. I will say trust your instinct, try to get the physio so you can feel like you are doing something, hope for the best, but be aware cp is in the main ok.