Stay strong, at age 5, they either hit the ball poorly, or very poorly, so it doesn’t really matter what side they are on Some let their kid hit off both sides at this age, but personally, I wouldn’t even let him swing righty
This might be overkill, but share this with your father in law:
Now, above all else, your kid has to love the sport. The 3 kids I know, right handed but all bat lefty (best hitters on team) LOVE baseball. If your kid is not having fun, and hitting lefty is adding to the unhappiness, you might have to reassess your master plan. Also, if after a few years, it just isn’t working out, it won’t be too late to go righty (seeing he’s a righty naturally). Ultimately, fun is the main thing, no use forcing anything since most kids don’t go on to play HS or college anyways.