My 8year old son has really bad shaped and yellowish teeth after the milk teeth fell out. What can I do to whiten them at least?

Pedodontic consult. Rather that rely on answers given to you by dentists over the internet, it would be prudent for you to consult with a pedodontist who can actually examine your son and suggest some solutions. Normally we do not recommend tooth whitening at this age. Perhaps a professional cleaning is all that is necessary. There might be some temporary measures such as composite bonding until he is older.
Best oral hygiene. Baby or primary teeth are often quite white compared to the color of the permanent teeth that follow. It's normal. At this age i wouldn't attempt anything to whiten them other than to make sure that the yellowing isn't due to ineffective brushing. Make sure that your child is brushing well and for long enough and limit consumption of refined carbohydrates and sugars.