[JaKe Elite]

This could work, a temporary fix (Creates a PVar storing player's last weapon they have hold before entering the vehicle) - The server then will force the player to hold that weapon whenever the player tries to switch for a new weapon.

PHP Code:

public OnPlayerStateChange(playerid, newstate, oldstate)
{
    if(newstate == PLAYER_STATE_DRIVER || newstate == PLAYER_STATE_PASSENGER)
    {
        SetPVarInt(playerid, "old_weapon", GetPlayerWeapon(playerid));
    }
    return 1;
}

public OnPlayerUpdate(playerid)
{
    if(IsPlayerInAnyVehicle(playerid))
    {
        SetPlayerArmedWeapon(playerid, GetPVarInt(playerid, "old_weapon"));
    }
    return 1;
}