I’d suggest that costs that aren’t clearly differentiated and attributable to the child are best shared to avoid a feeling of pettiness. For example, water and electric bills for a family of three are higher than for a couple without kids, rent for a larger place with an extra bedroom is higher, etc. but trying to assess how much extra the biological father should pay seems fraught with risk of coming across poorly.

On the other hand, things like clothes for the son, extracurricular activities, tuition (once he goes to college), etc. that are easily attributed to just the child are easier to leave as the responsibility of the biological parents.

Having said all that, there’s nothing to say that the stepmom can’t buy gifts, alone or shared with her partner.