If he was on 0.7 mL of a 150 mg/10mL solution three times a day he was being given 2.1 mL in total which works out to be 6.3 mg/kg/day (150 mg/10mL is 15 mg/mL so 2.1 m/L x 15 mg is 31.5 mg a day which works out to be 6.3 mg/kg if he was 5 kg). Therefore if he is now 6.75 kg he needs 6.75 x 6.3 mg a day which is 42.5 mg. So 42.5/15 (mg/mL) is 2.833 mL in total. 2.83/3 is 0.94 mL three times a day.

So 0.9 mL three times a day is roughly the same dose as he was initially prescribed (very slightly lower). Please note though this is just the maths of a dosing solution I am not a Dr so don't know if you need to adjust the dose for any other reason. Do call a Dr to check this is how we work out doses at work but we are using experimental drugs so can't refer to a medical professional for side effect advise.