If you're treating your son as a resident of MD for the whole year, all of his income is taxed to MD. This could put him in the top tax bracket for the state. Since he only worked in OH and CT for part of the year, his income from those states is likely taxed at lower tax brackets.
If that's the case, it makes sense that he'd have a balance due to MD even with the credits. The credits are limited to the tax that he paid in the other states but it won't cover the full amount of tax on that income since the MD tax rate would be higher than the tax rate paid in OH and CT.
