what's good about asa is he can pull off 15, which justifies a slimmer spider-man, and allows the audience to see him physically develop over the next 5 or so years. By the time infinity war hits, Asa could be extremely fit and muscular and it will make the payoff all the more enjoyable.
that said, if he is still too thin for civil war, give him a muscle suit and/or muscle shading. hollywood magic is almost limitless.