Would sending him to prison make him less likely to do it again? No
Would sending him to prison deter others rom doing the same thing? No (see my previous post)
Would sending him to prison cost the taxpayer one heck of a lot of money? Yes
Do we actually have any spare space in our prisons? No
If he were sent to prison then it would serve no useful purpose, and it would take the father and husband (no matter how fuckwitted he may be) away from the wife and son still reeling from the death of their daughter and sister.
I think something non-prison related might be more appropriate -- say a lifetime driving ban (as he clearly has no concept of road safety). Or community service if there were something appropriate.
He broke the law and therefore needs to face the consequences, yes. In our legal system those consequences can include a suspended sentence. There is no mandatory minimum sentence for manslaughter.
Whether I would want someone jailed who killed my child should be irrelevant to whether they actually are jailed. That's why we have a system of state-administered criminal law rather than a system of vigilante justice. It should be still less relevant to whether someone who kills someone else's child under different circumstances should be jailed.
I cannot think of one single useful purpose that jailing this man would achieve.