Morphologically, we have CeRC here (R stands for the resonant).
It’s an athematic noun, which means that endings were added directly to the stem (sunu-) without an intervening thematic vowel. Obviously, it is also an u-stem.
All athematic nouns have strong (e.g. nom. and acc. sg., nom.pl.) and weak cases (the rest).
If you subscribe to accent and ablaut paradigms, this particular noun is proterokinetic (or proterodynamic), which means that in strong cases the accent was on the full-grade root (the e-grade), whereas in weak cases both the accent and full grade shifted onto the suffix, so the u-suffix becomes –eu- (or –ou-, the o-grade) in weak cases. So, at this stage, we reconstruct NOM.SG. as *seunus - the standard citation form is the e-grade.
However, in the (PIE) daughter languages the root didn’t ablaut already, cf. Sanskrit sunus, Lith. sunus etc. (sorry, no diacritics).
Now about the root vowel. Usually the long u is not reconstructed as a vowel phoneme in the PIE. Since in Sanskrit and Lithuanian we have a long root vowel – and “y” in Slavic, we need something to make our short u long. If you subscribe to the laryngeal theory, obviously you propose a laryngeal H (capital H means unspecified); some researchers propose h3 here. So, now, at this stage, we reconstruct NOM.SG. as *seuHnus (or *seuh3nus). For weak cases, we could reconstruct the zero-grade stem *suHn-eu or rather *suHn-ou (other variants: *suh3neu, *suh3nou). Here's a partial paradigm for this noun (singular):
IE *séu̯h3-nu-s ‘son’
(Herzenberg 2010: 69)
If you don’t subscribe to the laryngeal theory, you could use a schwa instead of the laryngeal or simply reconstruct *suənu-, without any ablaut.