What Does My Liver Ultrasound Scan Report Indicate?

My son is just turned 18. His ALT has tracked from 56 in Feb 2016 (when he started on Ritalin during school) to 91 (June 2016), 75 (Dec 2016), 70 (Feb 2017), 104 (Mar 2017), and 138 (Apr 2017). In Mar he had a liver ultrasound which showed a small amount of Fatty Liver . During Dec-Mar his weight increased from 80 to 85.7kg; he is 5 3.5 . He has started on a no-carb/no-sugar diet and ramped up his exercise (he was out for 9 weeks during Sept-Mar with 3 bouts of mild concussion from sport). He is now back down to 80.8kg after 2 weeks and will continue to progress towards c.70kg over time. My question: How long does it take for the ALT readings to respond to these diet / weight changes?