Dear T.,
I think your best solution would be to first have a conversation with your boyfriend about the values you both want to instil in your children. This can be a relatively neutrual ground to get that talk going. I'm sure that you will both have similar ideas including a good sense of respect for adults and other children, high self-esteem and even good health.
Then it is just a matter of setting goals and taking action. Manners are the building blocks for respect for others. If you like, you can require all the children to say please and thankyou etc. This can begin to help with his eight year old's rudeness.
Good health goes hand in hand with plenty of exercise. With the over-full plate most parents have these days it can be hard, but I would plan trips to the local park or at least get out there in the back yard with them for 30 min to an hour each day. That way, they can expel energy out side and perhaps be less rambunctious in the house. It will also help with any excessive TV watching.
By working on these issues together, you and your boyfriend will become closer and present a united front to the children. My guess would be that the 8 year old is manipulating things a bit. He knows picking on your child will iritate you and he also knows dad will have to be on his side in order to reassure him that he's important, (i.e. more important than you). For him this is a contest of wills, and he is trying to out win you in every battle.
Also, if you and your boyfriend can spend quality time with all three children together, he is more likely to catch on to this mischevious game and see your viewpoint more clearly.
However, I also want to encourage you to take up more time with his children. Your boyfriend has opportunities all week to conect with your child, but you only see his on the weekend. When you call his children 'timid' and 'weird' it shows that you have a negative image of them and are less likely to accept them for their faults. Just as you want your boyfriend to accept that your two year old is loud when he plays with others, you should be willing to accept his children.
If you can manage the difficult task of meshing your two families together it will be a great accomplishment, but it will take effort on everyones part. I think in the end though, you will reap many benefits. And also remember, don't allow eachother's children to become tools in this disagreement with your boyfriend. Children deserve nothing but love, though love does come with swift discipline.