As it happens, every spell that involves a melee spell attack also requires a somatic component. From the Player's Handbook:
Spellcasting gestures might include a forceful
gesticulation or an intricate set of gestures. If a spell
requires a somatic component, the caster must have free
use of at least one hand to perform these gestures.
In order to cast these spells, your character would have to release the target with one hand, make their gestures, and then make the touch attack. Depending on the spell in question, the somatic component could be the touch attack itself, but regardless you cannot just hold a target with both hands and then cast the spell.
If you would like to know how to arbitrate circumstances that may grant a caster advantage on their melee spell attack, I'd invite you to look at the various conditions for an idea of what to expect. A brief summary of the conditions from the PHB to help you along:
Your Target is Blinded- Provides advantage on the attack rolls.
Your Target is Grappled (whether by you or someone else)- Does not provide advantage on attack rolls.
You are Invisible- Provides advantage on the attack rolls.
Your Target is Paralyzed, Petrified, or Prone- Provides advantage on the attack rolls.
Your Target is Restrained (whether by you or someone else)- Provides advantage on the attack rolls.
Your Target is Stunned- Provides advantage on the attack rolls.
Your Target is Unconscious- Provides advantage on the attack rolls.
One final note is the big exception to this rule. A sorcerer can cast a Subtle Spell, which requires no somatic component- and thus does not require a free hand to cast, so you could indeed cast the spell while holding your target. In such a case, it's the DM's call whether the circumstance provides you with an advantage on the attack roll or whether it automatically succeeds... though since you don't necessarily auto-hit on even an unconscious person, advantage on the roll is the most likely result, and would presumably be worth the Sorcery Point spent to obtain it.