i've made my own 'lean' but not with codeine, rather i made it with morphine but essentially it should be the same, get somthing sweet that he likes and basically crush them into that, make sure to add ALITTLE vodka (i say vodka because its the least appearent hard liquor to mix into stuff, and i mention this because without adding just abit of alcohol to somthing like this is that it'll start to deteriorate somewhat quicker so the vodka'll preserve it and give a decent addative effect but you want to make sure you fix it up so that he can get a good high but not die too, so if he's opiate naive i probably would max fix a 500ml bottle up with some beverage and eesh, i'm not codeine knowledgable anymore because i'm on high dose daily stuff now but, i remember 200mg when i was 15-16 would floor me and have me nice and good so i suppose you can get a jumping off point from that, also i don't know if 400mg is really a ceiling dose so you can try researching that because they say 400 is when your body stops converting codeine to morphine but i've read contradictory stuff about it, i do eat 90 codeine tabs within a week every month now though i never really think about what dose of it i'm taking because of my tolerance, but you should be careful with how much you decide to mix into the beverage, and just warn him to take a few shots worth maybe make him a 750ml bottle and tell him to go as needed through the day but can't you get him dihydrocodeine in the UK aswell? i think thatd be a better suited proper lean beverage
edit - also only add 1 shot of vodka (50ml) per 500 ml you make that should be more than enough at 10% alcohol which is usually whats in the typical cough syrups anyways
maybe make a 500ml bottle of a drink w 400mg worth of codeine so there should be 1.25mg/ml worth