How to restrict outbound EC2 to only access S3?

You may want to use VPC endpoints here.

http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/vpc-endpoints.html

"A VPC endpoint enables you to create a private connection between your VPC and another AWS service without requiring access over the Internet, through a NAT instance, a VPN connection, or AWS Direct Connect. Endpoints are virtual devices. They are horizontally scaled, redundant, and highly available VPC components that allow communication between instances in your VPC and AWS services without imposing availability risks or bandwidth constraints on your network traffic."

is it possible to just allow the EC2 instance to access the S3? Is there any ip connected to the S3 bucket or can I set one?

S3 uses many IPs. I suspect it would be difficult to nail down a list of them all. Additionally, there is no IP-to-bucket mapping, and it is not possible for you to specify an IP for a bucket. S3 is a managed service that AWS runs, and they have sole full control over their IP address usage for the service.

If you need to filter at this level, the easiest thing to do is to use a forward proxy (like squid) with a default deny ACL and then allowing only access to the S3 domain.