Does my paternal aunt's son from her first husband become a brother to my son, and his paternal uncles become paternal uncles to him, if my paternal aunt breastfeeds my son along with her son from her current husband before he completes two years of age, and is it permissible for me to uncover before my son's foster brothers when they reach puberty, given that I did not breastfeed any of her children?

If two children suckle from the same breast, they become siblings through suckling (rada'a). Based on this, your cousin's son from her first husband is considered a suckling brother to your son, due to their sharing your mother's (your cousin's) breast.

As for the uncles of your cousin's son, they are not considered uncles to your son, because he did not suckle from their brother's milk.

Your son's suckling brothers are not mahram to you, and you must wear the hijab in their presence when they reach or are close to reaching puberty.

It is a condition that there be five full sucklings for the prohibition (of marriage) to be established.