I guess you know that the result can be written as a polynomial.
$$w_0+w_1{\cdot}r+\dots+w_n{\cdot}r^n.$$
So your question is how to estmate the coefficients $w_i$; they are called "Cross-sectional measures", they can be defined for any convex body $K$. 

 - $w_0$ is the volume, 
 - $w_n$ is the volume of unit ball.
 - $w_1$ is the area of the boundary, I think there is no good formula in general, but estimates are known and you could write it as an integral.
 - I am sure that there is no good formula for the rest of $w_i$.

If you want to write $w_i$ as an integral, 
check for example Burago--Zalgaller, Geometric inequalities. 
Fore example 
$$w_i=\mathrm{MayBeAConst}{\cdot}\int_{\partial K} \sigma_{n-i}(k_1,k_2,\dots,k_{n-1})\\, d\mathrm{area}.$$
where $\sigma_i$ is the $i$-th elementary symmetric polynomial and $k_i$ are principle curvatures.
In your case it is easy to find $k_i$...