Your mother may live long and you may depart first...in that case your intention will not be actualized. You will be sorry about not entering the buddha way and your aged mother will be responsible for preventing you from doing so. Both of you will be at fault and will achieve no good result.
On the other hand, even if your mother dies of starvation, letting her only son enter the way will be greatly beneficial to her own attaining of the way. You have been unable to discard family obligations and attachments through many lifetimes. Doing so now and meeting the buddha-dharma truly repays your parents' kindness...it is said when one child becomes a monk the parents of seven generations attain the way. Don't lose the opportunity of attaining bliss for countless eons through attachment to this present drifting life. You should carefully consider this.