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To express "with" in the sense of "accompanied by", we can use just a third ending —

putreNa gacchati पुत्रेण गच्छति "(s)he goes with (his / her) son"

but in that case it is better to add **saha सह.

putreNa saha gacchati पुत्रेण सह गच्छति "(s)he goes with (his / her) son"

Instead of using third ending, we may make a descriptive /samAsa with sa स (a shortened form of **saha सह), and make the samAsa linkto whoever is accompanied —

saputro gacchati सपुत्रो गच्छति "he goes with (his) son"

saputrA gacchati सपुत्रा गच्छति "she goes with (her) son"

Of course the case and gender of the descriptive must be the same as the case and gender of the accompanied person —

saputrAm apazyam सपुत्रामपश्यम् "I saw her accompanied by her son, I saw her and her son"

tAM saputro 'pazyam तां सपुत्रो ऽपश्यम् "I accompanied by my son saw her, I and my son saw her " (male speaking)

tAM saputrApazyam तां सपुत्रापश्यम् "I accompanied by my son saw her, I and my son saw her" (female speaking)