Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.

I think there are two reasons why his mother rejects this marriage. First, there is a type of unlawful relationship which is going on. Second, you are two years older than him. Islamically, a Muslim should not get married to someone who is not chaste unless he or she makes a sincere repentance to Allah. For getting married to a divorced lady, there is nothing wrong about it. The Prophet got married to Khadijah who was older than him and she has previously married before him. Allah issues a general command to get married to those who do not have wives or husbands from both male or female, “And marry the unmarried among you and the righteous among your male slaves and female slaves. If they should be poor, Allah will enrich them from His bounty, and Allah is all-Encompassing and Knowing.” (An-Nur 24:3).

All of the wives of the Prophet (peace be upon him) were divorced except Aisha  and widowed except Aisha and Zainab  and the Sahaba would marry divorced/widowed women as well.

An important point to note is that when Umm Salamah was widowed, Abu Bakr, `Umar  and the Prophet  all proposed to her very shortly after her husband’s death (Abu Bakr and `Umar got rejected obviously).

Allah Almighty knows best.