Hi Lee Anne,
Yes, you could do 8mL per day for that combination of milk thistle and dandelion. You can also increase that dosage if needed to 15mL/day if it seems like he’s needing a little more. You can certainly add turmeric tincture to the blend if he’s not wanting to do capsules! Just note the therapeutic dosage for turmeric tincture is quite high. What strength do you have at home? It may be helpful to do a separate tincture of just Turmeric.
All herbs that you have mentioned are safe long-term, so it would be helpful to stay on for at least 2-3 months while observing and re-assess at that time. The infusion of nettle and st john’s wort is safe long-term as well, just note the photosensitivity risk with high doses of st john’s wort (although I am assuming it won’t be a high dose in an infusion!)
Warmth,
Riley