This time of year you would normally be worming for encysted redworm and tapeworm, neither of which reliably show up on worm counts. So I would worm, but with a horse that is dropping weight I def wouldnt use a combined product (as its a bit powerful if there is any underlying reason for the weight loss), single product for each with at least 2 weeks in between (eg Equest and Equitape would give you the same active ingredient as Pramox would, but safer for horse).
However, if the weight loss is unexplained, and assuming horse is eating well and getting good amount and quality of feed, and given theres a blood test for tapeworm to see if they actually need worming for that, I would be tempted to get vet to take bloods, that way they can check for anything underlying health wise and also send off part of the sample for tapeworm testing, killing two birds and all that. They will also be able to advise whether encysted redworm wormer would be a good idea right now, or whether to wait for blood test results to come back. My big chap who looked very poor and I thought it was just rubbish hay one year actually had a very low white cell count due to a recurring virus, so it was well worth while getting the blood test.
NB Make sure horse doesnt have flu jab close to worming or until vet has advised if thats coming up - the combination of flu jab and wormer was implicated in my chaps illness and he can no longer have the flu jab.
