A search for lactulose turned up this, under Important Information: "You should not use this medication if you are on a special diet low in galactose (milk sugar)." Which sounds to me like you shouldn't use the drug if you can't tolerate milk sugar. But I could be misinterpreting the warning.
Personally, I've found chia seeds to be very effective in treating constipation. Another internet search suggests that they are safe for cats to eat, so if it were my kitty, I'd try mixing a few of the seeds (maybe 1/8 teaspoon to start with?) in with Benji's wet food, then give it a day to see if that helps.