For the cost of warptime and meltabombs, you could get doombolt, nurgle's rot, and a familiar. This will give him some range while footslogging, and something to do while in assault. It's pretty fluffy too.
I like the looks of this. Should go well with support from your buddy.How about this? Terminator armour makes your Sorceror a bit more survivable (plus, it's just cool)...
• Filthbringer, Sorceror of Nurgle - Terminator Armour, Mark of Nurgle, Doombolt, Nurgle's Rot, Familiar
• 5 Plague Marines, squad 'Bubos' - Plague Champion with powerfist; 2 meltaguns, Rhino
• 5 Plague Marines, squad 'Poxus' - 2 flamers