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A TBI brain is like other brains; if you exercise it and stretch it, it remains plastic and grows. There are workarounds for everything, but he needs tp put in the effort. Having something to motivate him should help. I'm nearly 30 years out, and I can still "grow" when I make the effort - which I do every day.
Most things are a bell curve in recovery... and ageing also has its physical toll too. The physical weaknesses caused by injury (in mine bad balance & easy break bones) means physically i am more disabled.... but now 60 and still learning new stuff 53 yrs later. The last year saw me learn the piano to a reasonable level - able to do singalongs and 'pub' gigs.