You have some options and I think you will be able to make pumping work.
As has been explained, you could reduce basal units by 20 per cent and be able to program 0.10 per hour which would accumulated to less than the 3 units of Lantus. @Tim35 further explains that you can set zero basal rates so that allows for a further reduction of .80 (if there are 16 time periods and you use half of them to alternate zero basal, so you’re down to 1.6 units or just more than half of what your son currently uses.
My son’s experience when transitioning from injections to pump therapy is the opposite of what others are describing. He was using less than 2 units of insulin a day including bolus insulin for meals. We had similar concerns to yours about being able to schedule basal low enough. He uses the OmniPod which goes down to .05 units/hour for basal and does not allow for zero basal. We expected that we would have to use diluted Humalog in order to make pumping work. However, his insulin needs just about doubled when using only fast acting insulin so we never needed the diluted insulin. This is an option for you if the programming using zero isn’t enough. You may find his insulin needs increase rather than decrease though.
The most sensitive time for Caleb has been and still is the wee hours of the morning when he is sleeping. Although his insulin needs increased on the pump, his basal needs were not proportionate over 24 hours and I would have to get up and set a temp basal of zero for those early morning hours. An alternative would have been to target a higher bg when he went to bed, I opted for manually setting zero basal. With the t slim, you could accommodate something like this with the basal program of zero, which is nice.
As I recall, Caleb’s pump basal rate was estimated as a straight-line allocation of his injection basal without any adjustment. It’s interesting to me to hear that this is sometimes adjusted for a reduction.
Good luck! I feel pretty good about you not having to return the pump.